Hybrid & Electric Vehicle High Voltage Isolation Fault Systems

Editor's note: This article was originally published Dec. 30, 2014. Some of the information may no longer be relevant, so please use it at your discretion.

High Voltage (HV) systems that are resident on hybrid, electric and fuel cell powered vehicles provide many advantages, relative to improved fuel economy and enhanced flexibility of propulsion modes to acquire the improvement of fuel economy. The HV systems are mounted to the vehicle body/chassis and there must be a minimum electrical isolation (resistance) maintained between the HV system and the body/chassis to ensure a safe vehicle during operation or repair.

Because the automotive industry has adopted that any voltage greater than 60V as the threshold for HV and federal government regulations require the OEMs to monitor the chassis for HV leakage to the chassis, the OEMs must provide safety systems and sensing for any vehicle with systems that are operating at voltages greater than 60V.

The HV systems offer new testing and diagnostic challenges to the industry that need to be understood by the automotive technician to ensure any Loss of Isolation (LOI) problems in the HV system can be identified and repaired. The LOI on electric powered vehicles is one of the more common failure modes, irrespective of manufacturer, and it is critical that technicians are aware of the systems that sense these failures. This article will explore how the hybrid system monitors the body/chassis to determine if the isolation resistance has been lowered/reduced, between any HV system component and chassis, compromising the isolation and causing a HV LOI. This article will also cover systems that are embedded in the vehicle systems software to test the HV system and manual testing methods that can be used by a technician to test the HV system to determine where the fault is/has occurred so it can be repaired.

For most hybrid and electric vehicles, the HV component family typically includes the battery pack, power inverter, electric-machines (MGUs), dc-dc converters, and in most cases an electric air conditioning compressor. Other HV systems, such as electric heating systems (e.g., PTC heaters), also would be considered part of the HV component family. This includes the control system for each of the family of components

As hybrid electric vehicles (HEV) and its derivatives continue to populate the market, HV LOI failures have become a common failure mode for technicians to analyze, diagnose and repair. There are currently more than 70 different HEV models in the market today that will need service; many of them are no longer under warranty. The 70 models referenced do not include battery electric (BEV) or plug-in electric (PHEV) vehicles. The LOI failures and how the HV systems controllers detect these failures is not well understood in the automotive repair industry. Therefore, this article will dissect the HV related controls and diagnostic systems to help simplify how the HV systems operate and detect LOI that will help assist technicians in understanding how these failures occur, and how to test the HV component systems to locate the source of the LOI.

What Is isolation and an isolation barrier?

For the purposes of this article, Isolation will be defined as an electrical resistance barrier that exists between the HV components and the vehicle chassis. Although HV components use a lower voltage (12V) controller to control HV functions, there must be an isolation barrier maintained between the HV system components (including the 12V controller) and the chassis to maintain a high level of safety while operating or repairing the vehicle. The HV system is considered to be a floating system. Specifically, the HV components fasten to the chassis but, the HV components do not use the chassis for ground.

However, the 12V controllers do use vehicle chassis for ground. The HV battery pack or power inverter serve as the power and grounding points for the HV system (depending on mode of operation), not the ehicle chassis. Therefore, with respect to the vehicle chassis, the HV components are electrically connected in parallel. These components are fastened/mounted to the chassis but, electrically float on the chassis because, they do not use the chassis for grounding. Therefore, an electrical resistance barrier between HV current and the chassis must be maintained to ensure safety for anyone interfacing with it.

To simplify and understand the circuit, using Ohm’s or Kirchhoff’s Laws for parallel circuits can be applied to determine how much HV electrical current would be injected onto the chassis from the HV system.

If there must be a floating resistance barrier between the HV system and the chassis, then how much electrical current is permissible? The answer lies in the Federal Motor Vehicle Safety Specification (FMVSS) 305 specification. According to the FMVSS-305 specification the minimum isolation resistance barrier that must be maintained between the HV system and vehicle chassis is 500Ω/V(i.e., 500 ohms per volt). For example, if the vehicle system operates at 300V dc then, the calculation for the isolation barrier would be: (300) (500) = 150k (kilo)Ω (150,000Ω). In theory, if the resistance barrier were to decrease to a level ≤150,000 then, a Diagnostic Trouble Code (DTC) would be triggered and the MIL would illuminate. Example names of DTCs associated with LOI are Loss of Isolation or High Voltage Leak.

In practice, OEMs will calibrate the software at a much higher resistance barrier level to ensure that the chassis electrical current never reaches a dangerous level. For example, to ensure safety, the OEM may calibrate the DTC level at 200,000Ω instead of 150,000Ω. What is the danger point? The maximum permissible electrical current is 4 milliamps (mA). Although currents that are less than or equal to 4mA will provide some very noticeable “feel” of electricity and could cause discomfort, it is not fatal.

Let’s consider the previous 300V dc system example. Using Ohm’s or Kirchhoff’s Laws, the electrical current can be calculated to determine the maximum permissible electrical current between the HV system and chassis. The calculation would be: 300V/150,000Ω = 2mA of electrical current. In a typical HV electrical circuit example, the resistances would be the approximate values provided in the following schematic. Notice that the highest resistance values are 150 Megaohms (MΩ) and the lowest is 1MΩ. There are four components in this schematic connected in (resistive) parallel to the chassis. Therefore, according to Ohm/Kirchhoff, the total resistance of the circuit is approximately 825kΩ or 825,175Ω of resistance barrier between the HV system (bus) and the chassis.

If this were a 300V system, the total electrical current flowing into the chassis from the HV system would be approximately 372μA (microamps), based on the 825,175kΩ resistance of the entire parallel circuit. This is well below the maximum permissible value of 4mA.

However, if the electrical resistance barrier of one HV electrical component were to decrease (due to an electrical/electronic failure), the entire balance of the resistance barrier would change.

rforming some simple math would indicate that the total resistance of the HV has decreased to approximately 29.8kΩ and the electrical current flow is approximately 300V/29.8kΩ = 10mA. This level of electrical current flow is 2.5 times higher than is permissible by the FMVSS-305 requirement. The 10mA electrical current level is enough to begin causing extreme pain and possible damage to parts of the human body. As a reminder, the only HV components that use the chassis for ground are the HV component controllers. These controllers use the 12V vehicle system for power and ground but, will control the HV components by using components designed for HV isolation. The controllers contain circuits that will electrically isolate the HV system sensing from the 12V logic system to maintain a resistance barrier. Therefore, a HV controller could also be the source of an LOI.

Because the HV components are mounted to the chassis, there is always an opportunity for HV to become electrically present on the chassis due to the failure of a HV component to maintain its isolation barrier. If a resistance barrier is not maintained between the HV system and the chassis the possibility would exist for the vehicle operator or a technician servicing the vehicle to contact HV, with the chassis being the medium for carrying HV electrical current. Therefore, if a loss of isolation occurs, the chassis becomes the medium in which electrical current can be present, and electrically “shock” an unsuspecting owner/technician servicing/repairing the vehicle – assuming that the proper safety equipment and precautions are not employed.

To minimize/eliminate the possibility of being shocked or injured from an LOI there are requirements that were instituted decades ago to ensure vehicle safety systems monitor the chassis for LOI “leaks” that are present on the chassis. The FMVSS- 305 requirement governs how LOI is to be monitored. To paraphrase the FMVSS 305 requirement, it summarizes that the vehicle chassis must be continually monitored for LOI to ensure occupant safety anytime the vehicle systems are operational. Manufacturers have taken this requirement one step further by assigning diagnostic trouble codes (DTCs) and, in some cases, data parameter identifiers (PIDs) on the scan tool to assist vehicle operators and technicians in identifying and repairing LOI failures.

re are some manufacturers that have developed software routines or automated procedures to assist technicians in locating a component that has an LOI failure. However, in most cases, the technician will be relegated to finding the actual LOI fault without the assistance of software or special procedures. The LOI failures can be categorized as purely resistive or capacitive. Resistive failures can be defined as HV components that are physically making a connection to the chassis through direct contact or through a sub-component that has a physical failure. Resistive failures are the easiest failures to locate because, the failure is typically constant and there are no other variables that determine its level of failure of when it occurs. The other failure mode can be classified as capacitive.

These faults that can be classified as intermittent or require special circumstances to find the LOI fault. The difficulty factor in locating capacitive LOI faults is much higher than resistive faults. Capacitive failures can be faults of capacitor components within a HV component or failure of a component (or the component sub-assembly) that is capacitive in nature – meaning that the failure “functions” like a capacitor when electrical current is transmitted through it. Some of these failures can be extremely difficult for the vehicle diagnostic system to locate, and some test protocols to find capacitive LOI failures must be executed with the HV system disabled (powered OFF/vehicle shut-down), due to the necessity of eliminating high levels of electrical noise that, make it difficult for the diagnostics to find any LOI failure.

Hard (constant/continual) LOI fault conditions for extended periods are much easier for the diagnostic software to satisfy enabling criteria (e.g., faults are easier to find). In most cases, to ensure that the FMVSS 305 requirements is fully met, OEMs will institute a control software strategy so the vehicle will not operate the electric propulsion system when/after an LOI has been detected. In most vehicle systems since 2004, when a LOI is detected during vehicle operation the system will log the failure, turn on the malfunction indicator lamp (MIL), store a DTC, and disable any HV electric propulsion system operation until the failure has been repaired.

As an example, if a vehicle is operating when an LOI is detected, and the vehicle is then powered off to end the drive cycle, the next time the system is powered ON for a drive cycle the hybrid controller will not permit the HV battery pack contactors (relays) system to close to enable electric propulsion. The result is a loss of electric propulsion and HV operation but, the MIL will illuminate and a DTC will be stored. This will protect the vehicle operator and/or service technician from the reasons for the LOI. The safety system will not permit the vehicle to disable propulsion during any drive cycle. This poses a great danger to the occupants and other motorists. This is why the system will store a DTC and not disable the system until the vehicle has been powered OFF or, the gear selector placed in PARK and no vehicle wheel speed detected, to ensure the vehicle is not being operated in traffic.

For the service technician, the anatomy of how they can receive an electrical shock is very easy, if proper safety precautions are not observed. When servicing the HV system, the technician should be wearing Class 0 Electrical Gloves and disable the HV system to mitigate the possibility of electrocution. This is standard within all OEM service procedures to ensure safety. Because the vehicle chassis is always an element of the HV circuit, if an LOI exists and the technician is not wearing HV electrical gloves and has not disabled the vehicle, if the technician contacts the chassis they now become a possible parallel path element of the circuit.

In fact, HV current always is being injected onto the chassis by the HV system. It is the amount of electrical current that is the key variable. If the HV current on the chassis is in the microamp range the technician will never “feel” its presence and is safe. If the electrical current is greater than or equal to 2mA (assuming voltages are greater than or equal to 60) the technician will begin to feel the effects, although the level may not be near a fatal level. Therefore, if: 1) the HV system has not been disabled, 2) Class 0 Electrical Gloves are not being used, 3) an LOI is present, 4) the technician is touching the chassis, and 5) the technician then touches any open connection that is connected to the HV Positive or Negative bus circuit, there is a possibility of being electrically shocked (e.g., electrocuted) and it could be fatal.

The chassis merely serves as a point for HV current to enter/exit from the Positive or Negative HV bus rail. It should be mentioned here that, depending on where the LOI failure has occurred, the technician may electrically be more positive or negative, with respect to the chassis. Therefore, either the Positive or Negative bus could electrocute the technician. The following electrical diagram will help in understanding this concept.

HV Chassis current sensing circuits

HV system uses two types of circuits to monitor the chassis for LOI. The two circuits use direct current (DC) and alternating current (AC) to monitor for LOI. The DC circuit monitors the chassis continually for LOI, as mandated by FMVSS-305. The AC circuit functions only when the vehicle has been powered OFF. Each of these circuits will be discussed in detail within this section of the article. The DC circuit monitoring is typically performed by either the Battery System Controller or the Power Inverter System and Controller. The AC circuit monitoring is typically performed by the Battery System Controller. Each system will communicate with the Hybrid Controller (master controller of the hybrid system) via Controller Area Network (CAN) messaging so that the Hybrid Controller can safely manage data and functions of the hybrid system.

Chassis LOI monitoring using DC sensing

Itoring the chassis with a DC circuit is a simple method to determine if there is an LOI, and it is a simple circuit to understand. To simplify the DC LOI detection circuit, it typically resides in the battery or power inverter control system, uses a simple series circuit that connects the positive and negative HV bus rails by using two resistors. Each resistor is typically valued in the 1MΩ range to significantly reduce the current flow between the positive and negative HV bus for safety while providing excellent voltage fidelity for accurate measurements.

Although we consider the isolation barrier to be resistive, it should actually be considered an impedance barrier. Although we will not include a background section on electrical impedance, it is important for a technician to know the difference between resistance and impedance. Impedance is an AC electrical term that defines the effective resistance (or opposition) to oscillating currents in a circuit and circuit components, due to the inductive and capacitive properties of circuit components with respect to the vehicle chassis.

Because most materials contain capacitive and inductive properties, impedance is always considered when analyzing an electronic circuit. The inductive and capacitive properties are added with the pure resistive properties of the circuit and its components to determine the overall effective resistance. Inductance, capacitance, and resistance are added mathematically in a complex (square-root) mathematic function to determine the overall impedance (effective resistance) of the circuit. For purposes of clarity and thoroughness, the impedance equation (formula) has been included here.

Where:

Z = Impedance
R = Resistance
X_C = Capacitive Reactance
X_L = Inductive Reactance

The symbol for the units for the Impedance measurement is the letter "Z". Therefore, the reference to "Z" in the circuit encompasses all properties of the circuit that must be measured as part of the DC continual resistance measurement.

Applying the DC LOI Sensing Circuit to the Vehicle

Sensing "Z" with DC to locate an LOI is very similar to diagnosing an injector with a Digital Volt-Ohmmeter (DVOM). Although a standard DVOM can provide basic information about the circuit whether the injector is operating or not, it really doesn’t provide the diagnostic fidelity of an oscilloscope. The standard DVOM does provide data to detect a gross (general) circuit information but, it does not provide live data and visual waveform representation provided by an oscilloscope that results in data with a much higher fidelity (precision and accuracy). When analyzing the DC LOI sensing circuit, the analysis indicates that the circuit is functional only when the HV circuit is operational.

Note that the HV battery pack K1& K2 contactors (relays) are closed and all of the other HV system components are connected to the battery pack and the circuits are operational. Note that the controller circuit board is connected between R₂ and R₃. This mid-point serves as a reference point for the controller to measure voltage across R₂ and R₃. If the HV circuit has very low current leakage on either the positive or negative rails, the voltage drop across R₂and R₃ will be virtually identical. However, if either the positive or negative rails develop a more aggressive leak to ground (due to a cable, motor winding, component, etc.) then, the voltage drops across the resistors will become unbalanced, and the voltage will be shifted to be more positive or negative with respect to the mid-point. The more current leakage onto the chassis will result in a larger voltage imbalance across the resistors. When there is a voltage imbalance (whether on the positive or negative rail), and it finally equals (increases/decreases) the calibrated voltage threshold value in the software, the MIL will illuminate and a DTC will be stored in the controller to alert the technician that there is an LOI failure.

The DC testing cycle is accomplished by the controller very quickly. The DC test cycle can be accomplished by the controller in seconds, typically less than 30 seconds and it is a continual testing function when the vehicle is powered ON. Fundamentally, the DC circuit cannot indicate where the failure has occurred because DC circuit properties are a gross measurement circuit and cannot provide targeted diagnostic data unless there are automatic diagnostic software routines or special functions as part of a scan tool that can be activated to help locate the LOI. More importantly, it is very difficult (if not impossible) for the DC circuit to determine if there are capacitive LOI faults because, capacitive failures will block DC current. This will actually hide the failure mode. Therefore, to provide fidelity for locating or confirming an LOI failure, it will be necessary for the vehicle safety system to utilize an additional diagnostic circuit. This circuit will use AC to help detect LOI.

Chassis LOI monitoring using AC sensing

Sensing "Z" with AC to locate an LOI is a very different process when compared it to how the DC sensing circuit detects LOI. One of the primary differences between the DC and AC sensing circuits is the AC sensing is accomplished with the vehicle powered OFF. The primary reason that the AC sensing is performed with the vehicle powered OFF is to ensure electrical noise created by components in the electric propulsion system is deactivated. This will provide an electrically “quiet” environment for the controller (typically the battery controller) to begin measuring "Z" between the HV system and the chassis.

Because the controller uses electrical signals (sine waves) to measure "Z" all other components must be powered off so the electronic filtering can detect any problems with the isolation barrier. If the electric propulsion system were permitted to be activated during the AC measurement process it would be extremely difficult for the battery controller to filter all of the unwanted noise from the system to determine if there was a low "Z" measurement between the HV system and chassis. In fact, it is very difficult to determine if a battery pack system as an LOI using the DC method, due to high electrical noise activity. Furthermore, it is very difficult to sense battery pack LOI problems caused by capacitive failures when the HV system is operational, especially when batteries have capacitive electrical properties and the module construction can be of non-ferrous (metal) materials. This is also true for measuring the "Z" (capacitances) of HV components.

As an example of how the AC LOI system would operate, when the vehicle operator powers the vehicle system OFF, the battery controller will begin to measure "Z". This process can take several minutes to complete (e.g., five to 15 minutes). This is due to the AC measurement necessity for much more fidelity than the DC measurement. Furthermore, the AC measurement must filter unwanted electrical signals to ensure that an accurate "Z" measurement between the HV system and chassis can be accomplished. Without using an AC LOI sensing system, it would be impossible to locate “soft” or “hard” (intermittent or continual) LOI faults.

The simplest method to understand the AC LOI sensing circuit is to begin analyzing the circuit when the HV contactors in the open position. This is the natural state for the HV contactors after the electrical control system has de-energized the contactors when the vehicle HV system is powered OFF. The AC LOI method must be used for the battery pack because it is highly difficult for DC sensing to locate an LOI in systems or components with capacitive properties. Although, the battery pack is the component in that has received the most focus in this discussion, there other components that need to be tested using the AC LOI method such as the Power Inverter, DC-DC Converter, A/C Power Inverter, etc.

In the AC LOI control circuit, the battery controller will generate a low amplitude, low frequency AC signal and inject it onto the chassis. This AC signal is approximately a 5V sine wave with a low frequency (Hz) of approximately 2-5 Hz. The controller 5V sine wave is then transmitted to an amplifier stage that utilizes a Resistor-Capacitor (RC) network that is electrically connected to the chassis. The amplifier stage has two outputs. The first output stage, Voltage In (V_in), is measured by the controller and will be used as the reference waveform to ultimately compare the output measurement of the second output stage, using Voltage Out (V_out). The second stage will be connected in series with the vehicle chassis to acquire the total "Z" measurement. Since the RC network is located between the first and second stage, and the second stage is measuring "Z" of the RC network and the battery pack/vehicle chassis, it will generate an output waveform that is less than the V_in stage one output measured by the controller. If stage two waveform drops below a calibrated software value (due to low "Z"), a DTC will be stored for the LOI fault.

The primary reason that AC is used with an RC network is because, unlike using a capacitor in a DC circuit where current will be blocked by the capacitor, in an AC circuit the capacitor will permit current to pass through it. What results from passing current through both the resistor and capacitor RC circuit and the battery pack is the battery controller will be able to measure the "Z" of only the battery pack circuit to determine if there is an LOI. This can be accomplished because there is no other electrical noise in the system and the AC sensing circuit can use the properties of AC current to measure the capacitive isolation barrier between the battery pack and chassis – a test that could not be accomplished with the DC sensing circuit.

The capacitance being measured is the “Y” capacitance value. The “Y” capacitance is the capacitance measured between the HV system components and the chassis. Although there are other types of capacitances that can be measured, the “Y” capacitances in each of the HV components (connected in parallel) to the chassis will be the values effecting the total LOI "Z" value. When the capacitance values (one or more in parallel with the controller RC network) have reduced the "Z" value between the battery pack and the chassis, this will result in a lower overall Z value measured (with the RC network) by the battery pack controller and a corresponding DTC will be stored to alert the technician to the fault.

There are derivative diagnostics that can be performed by the vehicle safety system, depending on OEM, vehicle, and model year. There are several diagnostic routines that can be performed automatically by the battery or hybrid controller software or routines that are a combination of the controller software and special scan tool functions to initiate controller testing for LOI in components external of the battery pack. The OEM may elect, through a special procedure (without a scan tool or software) to provide a technician a procedure to place the vehicle in a specific operational state to help the technician test for an LOI condition. The vehicle service manual will provide additional information on any LOI special tests that the vehicle or scan tool will support.

The following sequence can provide an example of how sequencing battery pack, power inverter hardware, and A/C hardware components with software or scan tool special functions can provide additional testing methods and sequences to help a technician locate a HV LOI. All AC tests must be performed by the battery controller with the vehicle powered OFF:

Open K1 and K2 HV Contactors – inject/measure AC into battery pack to test it for LOI.
Close K1 Relay and Open K2 HV Contactors – inject/measure AC into negative bus rail to test the negative HV cable and power inverter circuit for LOI on the negative side of the HV circuit.
Open K1 and Close K2 HV Contactors – inject/measure AC into positive bus rail to test the positive HV cable and power inverter control circuit for LOI on the positive side of the HV circuit.
4. Close K1 or K2 HV Contactor and command either the positive or negative power inverter transistor motor drive network (one or the other at a time, not both at one time) circuit ON to test for any electric MGU winding LOI. This will test for gross electrical/electronic failures only, not intermittent failures caused by partial discharge energy that can only be generated at voltages greater than 300V.
5. The A/C circuit and the compressor motor windings can also be tested separately for LOI, much the same manner as the power inverter, by virtue of being connected to the HV bus and using its power inverter transistor network (this will test for gross electrical/electronic failures only, not intermittent failures caused by partial discharge energy).

By controlling the K1 and K2 HV contactors, many of the HV components can be tested for LOI, without the necessity of system disassembly. However, whether a system is able to automatically test or use special function tests with a scan tool to command the tests for LOI in a HV system, this functionality will be OEM dependent. However, if the vehicle is equipped with software that will automatically test the system for LOI (and provide the technician with some possible locations for the failure) or use scan tool commands to execute LOI systems tests, the technician will be required to confirm the LOI.

Confirmation means that the technician will be required to confirm an LOI DTC by using off-board testing and testers. Or, if the vehicle software is not designed execute automatic or scan tool commanded functions to help locate the LOI then, the technician must manually test each device in the HV system with an off-board tester to determine the fault origin. The OEM service information will provide the fundamental information on how to test individual components but, there are some “short cuts” that are technically very sound that can assist in reducing the amount of time in testing individual HV components for LOI. These short cuts are taught in hybrid courses by reputable hybrid technology training companies.

Manual testing for LOI using off-board tools

Although vehicle systems can utilize controller based DC and AC LOI testing, there will always come the time when manual testing of components will be necessary to either eliminate possible failures by process of elimination or confirm a failure. Although manual testing will be discussed in a more targeted focus in this article, each OEM will provide specific manual loss of isolation testing procedures for their system. The OEM procedures will typically write the procedures to test the insulation of a component for how well it isolates the circuits from the chassis by using the following process:

Wearing the proper personal protective equipment (e.g., Class 0 High Voltage Electrical Gloves) disable HV from the vehicle by using the proper disabling procedures.
Confirm the HV has been disabled.
Disconnect the HV cables that connect the HV components (if equipped with cables) to segregate them so that individual testing can be completed. The OEM service manual may provide a specific sequence in which to test the components. However, if a technician has received quality education and training and is familiar with HV hybrid/electric systems, they may elect to use quicker methods and procedures of testing the system.
Using a DVOM or Insulation Meter (e.g., Fluke 1587, Fluke 1507, etc.), select the proper insulation test voltage range to test the system. As an example, the Fluke 1507 will provide voltage ranges from 50, 100, 250, 500, 750, and 1000V.

NOTE: The OEM will state a voltage range for the technician to use for insulation testing. If the OEM does not require the use of an insulation meter to test for LOI, and there is no insulation test voltage range recommendation in their service information, there is alternative method that can be used. The alternative method is to determine the highest typical operating voltage of the system and set the insulation meter at the next highest voltage range. As an example: If the operating voltage range of the vehicle hybrid system is 120 – 170V then, use a range higher than 170 (e.g., use the 250V range). So, why wouldn’t a technician use 500, 750, or 1000V to test a 170V system? The reason is that a system with electronic and electrical components that are designed/rated to operate in a 170V system will get electrically stressed (and possibly damaged) if an excessive test voltage is used. Stressing a component means that a test voltage may exceed the operating maximum designed voltage specification of a device. If a capacitor is rated for 250V and 500V is used to test it, it will stress the capacitor and damage it immediately or reduce its operating life (e.g., it will fail at a later time but, much sooner than it’s normal service life). This general rule holds true for an electronic device or electric machine (MGU stator winding) tests.

When the electronic or electrical device is tested, the test results indicated on the insulation meter will be provided in units of Ω resistance. This provides an indication of how much isolation barrier (insulation resistance) is present between the device being tested and the vehicle chassis.

Motor-generator unit insulation testing example

To perform an insulation test on MGU 3-phase windings, the system must be properly prepared (see Steps 1 through 4 in previous section). Hybrid and electric vehicle MGUs are designed with 3-phase windings that share a common winding neutral connection. The insulation test for a 3-phase MGU is a very simple test. One of the insulation meter (black) leads is connected to chassis ground, and the other lead to one of the three MGU cables/wires that is usually connected to the power inverter.

After connecting the insulation meter to chassis ground, and one of the three MGU cables/wires, the technician will command the insulation meter to execute the test. When the test is executed, the insulation meter will inject a small current into the 3-phase windings to “excite” the chemical properties of the wire insulation and test the dielectric (isolation resistance) strength of the stator slot insulation and winding (enamel or varnish) coating properties. Although most insulation meters are capable of injecting a maximum of 2mA of current (at a voltage near or at the voltage range selected for the test), the actual current being injected into the windings will be in the microamp range.

This limitation of current injection into the three-phase windings is due to the isolation resistance barrier between the MGU and vehicle chassis. If there is a direct short between the windings and ground, the maximum current that can be injected is limited to 2mA by the tester control circuitry. This is a safety design feature to ensure that the user cannot receive an electrical shock that places them in danger of a fatal electrocution. The technician will execute the test until the maximum resistance value can be acquired by the insulation meter and will be displayed on the tester in units of Ohms resistance. There is no need to connect the insulation meter to the other two cables/wires of the 3-phase winding because, the windings are connected together at a common neutral point. Therefore, when the current is injected into one phase, it is electrically injected into all three phases for testing the isolation barrier between the MGU and the vehicle chassis.

Typically, an OEM will provide a minimum insulation resistance value for a component. As an example, the OEM may specify that the insulation resistance of a MGU should be greater than 10M (Mega)Ω. The Institute of Electrical and Electronic Engineers (IEEE) specifies in Standard 43-2000 indicates that, if a MGU operating voltage is less than 1000 V then, its insulation resistance shall be a value >5MΩ. Although the IEEE is the governing authority for testing of electrical machinery for all industries the technician needs to default to the resistance requirements of the automotive OEM in the service information. There are additional insulation meter techniques that can be used to help determine the condition of a MGU. If a technician has interest in learning these techniques, they should seek out a course from a qualified hybrid and electric vehicle training organization to learn the additional techniques and skills.

Power inverter insulation testing example

As with testing MGUs, when performing an insulation test on a power inverter system it must be properly prepared (see Steps 1 through 4 in MGU testing section). Hybrid and electric vehicle power inverter systems are designed to provide 3-phase electrical current to the MGU windings. Unlike the MGU, the three power inverter circuits are separate circuits within the power inverter that must be tested separately. There is no neutral (common) connection. The insulation test for a three-phase power inverter system is a very simple test. One of the insulation meter (ground) leads is connected to chassis ground. The three power inverter cables/wires will be probed and tested individually with the other insulation meter lead. Using the same testing procedure that was used on the MGU (selecting the proper insulation meter voltage testing range – e.g. 500V, etc.) each of the power inverter circuits will be probed and tested.

Unlike the MGU, power inverter insulation resistance is not governed by the IEEE, due to the volume of different power inverter applications and designs. Therefore, there is no single industry specification metric that can be cited. The OEM will typically provide a specification in the service information for this device. However, in the event that the OEM does not provide a specification, the typical insulation resistance for a good power inverter has tested at 500kΩ – 1MΩ. Conversely, a failed power inverter will easily be identified with insulation resistances that have been measured from 25kΩ (or less) to 100kΩ. It should be restated here, it is the FMVSS-305 requirement of 500Ω/V, and the associated calibration that, ultimately determines what resistance level will cause a MIL and DTC. Therefore, the FMVSS-305 requirement can (and should be) used as the governing baseline testing metric reference for all LOI testing because, it is electrically specific to all vehicle systems (irrespective of OEM). So, this is a wonderful metric to remember and use in your LOI testing.

Whether testing an MGU, power inverter, A/C compressor system, battery pack system, etc. using an insulation meter is a simple process of selecting the proper testing range and then, probing and testing each of the individual components for its insulation resistance (isolation barrier) value. Although a particular OEM and vehicle system may provide automated LOI testing or, a scan tool may also provide functions that will command the system to initiate special testing protocols, knowing how/where to test the system manually is an insulation meter is of utmost importance to a technician.