You need to be careful when working on the electrical systems of Hybrid vehicles (fig. 1).
In fact, you need to observe the same safety precautions as a commercial or industrial electrician. Working safely has two parts, and you are responsible for both parts: (1) having safe equipment and (2) using it in a safe manner.
Learn about the key safety features of the new Two-Mode Hybrid vehicles and the safety precautions you must use when servicing them. The GM Service Technical College is offering these training courses on-line at www.gmtraining.com. In Canada, these courses are available on the GM Pro Training website www.gmprocanada.com.
Course Title | Course Number |
High Voltage System Safety * | 18440.01W |
Two-mode Introduction and Safety | 18440.05W |
Two-mode 300v Battery System | 18441.01W |
* Required for all service personnel (U.S.) (including service manager, service foreman, service advisor, technician, body shop)
MULTIMETER SAFETY STANDARDS
Safety must be built into your multimeter. The International Electrotechnical Commission (IEC) recently defined new safety standards for test equipment. Equipment designed to the new IEC61010 standard offers an even higher level of safety than equipment designed to the earlier IEC348.
IMPORTANT: See the accompanying sidebar about the essential multimeter J-39200-A.
TIP: The IEC defines the standards, but is not responsible for enforcing them. Equipment manufacturers may self-certify their equipment, but you must look for the symbol of an independent testing laboratory to be sure that the equipment has been verified. Symbols of typical laboratories include UL, CSA and TUV (fig. 2).
VOLTAGE SPIKES -- THE INVISIBLE HAZARD
Hybrid vehicles produce electrical loads more complex and powerful than traditional automotive systems, creating greater possibilities of transient over-voltages (spikes). Motors, capacitors and power conversion equipment, all commonly used in hybrid vehicles, can be prime generators of voltage spikes.
When taking measurements on electrical systems, you can be exposed to the hazard of voltage spikes, which are invisible and largely unavoidable. They occur regularly on low voltage power circuits, and can reach peak values in the many thousands of volts.
Being affected by a voltage spike has nothing to do with misuse of the multimeter, because spikes can occur without warning. Your protection is dependent on the safety margin built into your meter. The voltage rating alone will not tell you how well that meter was designed to survive high voltage spikes.
Protection against voltage transients (spikes) is critical. It involves not just the maximum steady state voltage of the circuit, but the combination of steady state voltage and transient overvoltage capability.
OVERVOLTAGE INSTALLATION CATEGORIES
The most important single concept to understand about the new IEC standards is the Overvoltage Installation Category. The new standard defines Categories I through IV, often abbreviated as CAT I, CAT II, etc.
The categories are based on the fact that a dangerous high-energy transient will be dampened as it travels through the impedance (AC resistance) of the system.
Overvoltage Category | Description |
CAT IV | Three-phase at utility connection |
CAT III | Three-phase distribution, single-phase commercial lighting |
CAT II | Single-phase receptacle loads |
CAT I | Electronic |
A higher CAT number refers to an electrical environment with higher power available and higher-energy transients. For instance, a multimeter designed to a CAT III standard is resistant to much higher-energy transients than one designed to CAT II standards.
Within a category, a higher voltage rating denotes a higher transient withstand rating. For instance, a CAT III-1000 V meter has superior protection compared with a CAT III-600 V rated meter. The real misunderstanding occurs if someone selects a CAT II-1000 V rated meter thinking that it is superior to a CAT III-600 V meter.
WHEN IS 600 V MORE THAN 1000 V?
Within a category, a higher working voltage (steady state voltage) is associated with a higher transient, as would be expected. For example, a CAT III-600 V meter is tested with 6000 V transients while a CAT III-1000 V meter is tested with 8000 V transients.
What is not as obvious is the difference between the 6000 V transient for CAT III- 600 V and the 6000 V transient for CAT II-1000 V. They are not the same. This is where the source impedance comes in.
Ohm's Law (Amps = Volts/Ohms) tells us that the 2 ohm test source for CAT III has six times the current of the 12 ohm test source for CAT II. The CAT III-600 V meter clearly offers superior transient protection compared to the CAT II-1000 V meter, even though its so-called voltage rating could be perceived as being lower. It is the combination of the steady-state voltage (called the working voltage), and the category that determines the total voltage withstand rating of the test instrument; including the all-important transient voltage withstand rating.
TIPS FOR UNDERSTANDING CATEGORIES
Here are some quick ways to apply the concept of categories to your every day work.
- The general rule-of-thumb is that the closer you are to the power source, the higher the category number, and the greater the potential danger from transients.
- It also follows that the greater the short-circuit current available at a particular point, the higher the CAT number.
- The greater the source impedance, the lower the CAT number. Source impedance is simply the total impedance, including the impedance of the wiring, between the point where you are measuring and the power source. This impedance is what dampens transients.
MULTIPLE CATEGORIES
There's one scenario that sometimes confuses people trying to apply categories to real world applications. In a single piece of equipment, there is often more than one category. For example, in automotive accessory equipment, from the 12 V side of the power supply back to the battery is CAT II. The electronic circuitry (inside the PCM) is CAT I. In engine control systems, such as ignition controls, it is common to find electronic circuits (CAT I) and power circuits (CAT III) existing close together.
What do you do in these situations?
As in all real-world situations, use common sense. In this case, that means using a meter with the higher category rating. It's not realistic to expect people to be going through the category-defining process all the time. What is realistic, and highly recommended, is to select a multimeter rated to the highest category in which it could possibly be used. In other words, err on the side of safety.
OVERLOAD PROTECTION
Amp Inputs -- Use only a multimeter with amps inputs protected by high energy fuses. Never replace a blown fuse with the wrong fuse. Use only the high-energy fuses specified by the manufacturer. These fuses are rated at a voltage and with a short circuit interrupting capacity designed for your safety. Overload protection fuses protect against overcurrent.
Volt/Ohm Inputs -- The high input impedance of the volts/ohms terminals ensures that an overcurrent condition is unlikely, so fuses aren't necessary.
Overvoltage protection, on the other hand, is required. It is provided by a protection circuit that clamps high voltages to an acceptable level. In addition, a thermal protection circuit detects an overvoltage condition, protects the meter until the condition is removed, and then automatically returns to normal operation. The most common benefit is to protect the multimeter from overloads when it is in ohms mode. In this way, overload protection with automatic recovery is provided for all measurement functions as long as the leads are in the voltage input terminals.
WHAT TO LOOK FOR IN A MULTIMETER
If you are faced with the task of replacing your multimeter, do one simple task before you start shopping: analyze the worst case scenario of your job and determine what category (CAT number) your use or application fits into.
Choose a meter rated for the highest category you could be working in. Then, look for a multimeter with a voltage rating for that category matching your needs. (fig. 3).
TIP: For Two-Mode Hybrid trucks, a multimeter with at least a CAT III number is necessary.
TIP: Test leads should be certified to a category and voltage as high as or higher than the meter (fig. 4). When it comes to your personal protection, don't let test leads be the weak link.
In addition to the appropriate CAT number, look for:
- Meters and test leads with double insulation.
- Meters with recessed input jacks and test leads with shrouded input connectors (fig. 5).
- Test leads with finger guards and a non-slip surface (fig. 6).
- Meter and test leads made of high-quality, durable, non-conductive materials.
WORK SAFELY
Safety is everyone's responsibility, but ultimately it's in your hands. No tool by itself can guarantee your safety. It's the combination of the right tools and safe work practices that gives you maximum protection. Here are a few tips to help you in your work.
- Work on de-energized circuits whenever possible.
- On live circuits, use protective gear:
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- When making measurements on live circuits:
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- Thanks to Will Godfrey and Corey Glassman, Fluke Corporation
| ESSENTIAL TOOL J-39200-A Note the distinction between tool numbers. The original number was J-39200 and the present number is J-39200-A. The J-39200 has been an essential tool since 1992. The original tool was built to the IEC348 standard, not the present IEC61010 standard, which had not been established at that time. The J-39200-A presently being offered is built to the stringent IEC61010 standard, and is CAT III rated. The original J-39200 is not category rated. It does not have the overvoltage protection of a CAT III instrument, but is rated for use up to 1000 V. Of great concern, though, is whether existing J-39200 multimeters, after years of hard service, are still in good repair. Do they have high energy fuses? Are the test leads damaged, with exposed conductors? Are strain reliefs damaged, causing intermittent readings? Many -- probably most -- of these instruments have not been calibrated or serviced since new. This goes back to the earlier statement -- working safely has two parts, and you are responsible for both parts: (1) having safe equipment and (2) using it in a safe manner. If your old J-39200 is in doubtful condition, and you decide that working safely requires you to replace it, follow the guidelines presented above, and obtain a new instrument that is CAT III from Dealer Equipment, is such an instrument. |
