Standard Interpretations - (Archived) Table of Contents|
|NOTICE: This is an OSHA Archive Document, and may no longer represent OSHA Policy. It is presented here as historical content, for research and review purposes only.|
September 18, 1991
MEMORANDUM FOR REGIONAL ADMINISTRATORS ARA'S FOR TECHNICAL SUPPORT AREA DIRECTORS Through: Thomas Shepich From: David V. Loebach Subject: Field Service Memo on Electrical Testing EquipmentThe attached Field Service Memo entitled "Use of Electrical Testers by OSHA's Compliance Officers" is provided for the information of you and your staff. The memo is designed to inform CSHO's of the capabilities of various types of electrical measuring instruments and provides examples of measurements suitable for each instrument. It is intended to be a practical, hands-on document based on the specific instruments that OSHA owns. Please distribute it to each of the CSHO's in your offices. If you or your staff have any questions, please contact John Englert of the Cincinnati Laboratory at FTS 684-3721.
OSHA Cincinnati Laboratory
December 24, 1990
USE OF ELECTRICAL TESTERS BY OSHA'S COMPLIANCE OFFICERS
The purpose of this paper is to describe the various types of electrical testers currently available to 0SHA's compliance officers and to describe some common applications for which the instruments might be used.
There is a multitude of instruments that could be lumped into the general category of electrical testers. Generally this term would be so broad as to defy a comprehensive listing and application guide in a medium as short as a field service memo. However, the list of handheld testers typically used for compliance activities is more manageable.
LIST OF ELECTRICAL TESTERS USED BY OSHA:
1. Multimeters (volt, ohm, ampere)
a) handheld size
4. Battery Testers
5. Ground Wire Impedance Testers (Woodhead GLIT'S & ECOS 1020)
6. 120 VAC Receptacle Testers
7. Ground Fault Interrupter Testers
9. Electrostatic Meters
1O. AC Voltage Detector (TicTracers & Biomedical Field Probes)
Multimeters are the most widely used test instruments by electricians and electronic technicians. These meters typically measure AC and DC voltages; AC and DC currents, and resistance - hence the name multimeter. OSHA's compliance officers generally have multimeters that are relatively inexpensive, small in size with moderate accuracy and flexibility. Because multimeters are a mass market item, the unit cost of these instruments is relatively low so that some very economical units are still high quality meters.
a) Handheld Size Instruments The Fluke Model 21 purchased by OSHA is an example of an economical instrument that has good accuracy and some excellent features including autoranging, digital readout and touch hold. The compliance officer may use this instrument for safety inspections and other activities with a high level of confidence that the readings are sufficiently accurate for his applications.
The Fluke 21 is capable of measuring 750 volts AC and 1000 volts DC. However, voltages at these levels can be very dangerous and measurements should not be attempted above 120 volts unless the operator has been trained for such applications.
b) The pocket sized multimeter (Heath Model SM 2300) is simply a little more specialized version of a multimeter. Because of its reduced size, compliance officers can carry it on their person or in their briefcase all of the time much as they would ball-point pens. Although the Heath SM 2300 is accurate enough for most compliance activities, the small size of the meter can be a disadvantage for such applications. This is especially true of the short length of the test leads and the small size of the probes. We suggest that the pocket meter be used primarily to trouble shoot and test the compliance officer's own instrumentation and that the full function meter be used for compliance activities. It is recommended that CSHO's avoid using these meters for measurements above 120 volts.
The potential applications of a multimeter are so numerous that it is not possible to make a comprehensive list. Some typical applications have been listed in the appendix.
CLIP-ON CURRENT METERS:
Many multimeters provide ranges that allow current measurements without the use of separate probes but this requires disconnecting a circuit and reconnecting the multimeter in series with the circuit. This operation can be dangerous when dealing with power circuits and should be avoided by OSHA's compliance officers.
Clip-on current meters provide a means of measuring either AC or DC current without disconnecting the circuit. The clip-on probe is clipped around a wire without penetrating the wires insulation. The sensing of the current is achieved through inductive coupling. In many industrial applications, this is the only practical and safe method of measuring current.
Clip-on current meters come as complete self-contained instruments or as accessory probes for multimeters. Clip-on probes that measure AC current are relatively simple and inexpensive. Probes for measuring DC current are more complex and expensive.
OSHA's compliance officers are supplied a clip-on AC current probe as an accessory to their Fluke Model 21. This probe can be used to measure AC currents up to 150 amperes. The probe is used with the voltage measuring function of the multimeter (not the current mode), and it can be used with any multimeter that has an appropriate voltage range. Clip-on DC current probes have not been provided.
At resistances above 10 megohms (MQ), high voltages (500 to 1000 volts) are necessary to produce sufficient current flow to effect a resistance measurement. A voltmeter with a higher input resistance than a regular ohmmeter is also required. The megohmmeter or "megger" is an instrument which meets these requirements and is capable of resistance measurements to 1012 ohms (1 TQ). It is used for measuring insulation resistance and other high resistances. It is also used for continuity, ground, and short-circuit testing in general electrical power work. The basic circuit of a megohmmeter is similar to the ohmmeter part of a multimeter with the exception of the improved detector and high supply voltage. Older models use hand-driven voltage generators while newer models use batteries with step-up transformers.
Battery testers are typically nothing more than a simple voltmeter with some built in load resistors. The resistors provide a load for the batteries under test while the voltmeter measures the voltage drop across the resistor. The load resistors are sized to provide typical service loads for standard batteries; therefore, the instrument will have a selection of test positions that corresponded to the different battery types (usually only off the shelf consumer batteries).
The Simpson Model 379 battery tester is widely available to OSHA's compliance officers. It does not perform any function that couldn't be done with a multimeter and a few resistors. Its advantages are that everything is built into one package, and it's so simple to use that even the most unsophisticated operator should have no problems. The disadvantages are that it is much bulkier than the pocket sized multimeter and that many of the battery packs used in industrial hygiene instruments don't fall into the standard voltage or service load range so they can not be properly checked with the Model 379.
GROUND WIRE IMPEDANCE TESTERS (WOODHEAD GLIT's & ECOS MODEL 1020):
The ground wire impedance tester is a specialized device for testing the impedance of the grounding wire to 120 VAC receptacles. Most makes and models include a feature for measuring leakage current. Other features may be incorporated into the instrument such as wiring polarity checks (same function provided by simple receptacle testers).
The cleverness of the ground wire impedance tester is that it uses the circuit wiring of the branch circuit as a test lead back to the grounding bus of the circuit breaker box and it performs the test while power is applied to the branch circuit.
These units also come with an adapter plug for disconnecting the grounding wire of a piece of equipment from the grounding wire of the 120 VAC receptacle. This temporary test condition is necessary to check leakage current because it allows the tester to be connected in series with the grounding wire. The same adapter would allow an equivalent test using a full function multimeter.
Receptacle testers are very simple devices used to determine if a 15 or 20 ampere, 120 VAC electrical outlet has been wired correctly (hot to hot, neutral to neutral, ground to ground). These testers use a three light system that tells the user if power is present at the outlet and, if so, if the outlet is wired correctly. These indications are achieved by observing which lights are lit in the tester.
Receptacle testers do not perform any function that couldn't be done with a multimeter. Their advantage is that they are small, simple to use, easy to read and very low cost. They are also safer to use because they have the appropriate 120 VAC receptacle plug. If a compliance officer were to use a multimeter with only the standard test leads, he must poke a round test probe into a slotted plug. He would be increasing the risk of shocking himself or damaging the plug under test.
GROUND FAULT CIRCUIT INTERRUPTERS:
A ground fault circuit interrupter tester is a device which is used for testing ground fault circuit interrupters (GFCI). A GFCI is used to protect people who are using electrical equipment or tools. A GFCI unit is placed in-line between the electrical power source and the electrical equipment being used. The GFCI constantly senses the amount of current which goes out the "hot" wire and comes back through the neutral and compares the two readings. If there is a difference of more than a certain amount, typically 5 to 7 mA, between the two readings, the GFCI's internal breaker is tripped to cutting off power to the equipment. The presumption is that the mismatch of current between the hot and neutral wire is caused by a ground fault. GFCI testers are used to see if the GFCI is working properly. GFCI testers work by shunting a small amount current from the ungrounded "hot" wire to the ground wire. This causes the GFCI to sense a ground fault condition since no current is being sensed in the neutral wire. Certain testers use a fixed amount of current, typically 5 to 7 mA, to test the GFCI. Other testers can vary the amount of current shunted to the ground wire to more precisely determine the value at which the GFCI trips. It is important to make sure that GFCI units are working properly in order to avoid possible electrical shock hazards.
Handheld multimeters can not be used to perform the function of a GFCI.
Electrostatic fieldmeters measure the electric field potential surrounding a charged object. If a sufficient charge exists, there can be an electrostatic discharge which can be an ignition source for combustible gasses, damage delicate electronic circuitry, and can be dangerous to workers. By measuring the field at a given distance from the charged object, it is possible to determine the electric potential or voltage present at the surface of the object. Both the voltage (e.g. free electrons per unit area) and the quantity of charge (e.g. total number of free electrons on the object) that is held by the object are important. The quantity of charge depends on the object's size, shape, mass, the materials which make up the object, and the voltage on the object. Voltage is measured in volts and the quantity of charge is measured in coulombs. To measure the actual charge on an object in coulombs is difficult, but it is easy to measure the electrostatic potential (voltage) on an object. The voltage can be measured without touching the object; this is done by coupling the meter to the electrostatic field around the object.
Typically the electrostatic voltages a CSHO would be interested in measuring range from 500 through 30,000 volts. A quantitative measurement of electrostatic voltages is not possible using a multimeter for two reasons: 1) multimeters don't have high enough measurement ranges, 2) touching the test probe to the charged object would allow the electrostatic charge to discharge.
AC VOLTAGE DETECTORS:
AC voltage detectors are handheld instruments used for the detection of AC voltage without actually making connections or contact.
The TIF TicTracer is the AC voltage detector widely used by OSHA's compliance officers. The instrument is held in the operator's hand and is used to probe the wire or other object using the insulated tip of the instrument. This instrument detects the electric field set up around the object and presents an audible output when AC voltage is present. Other brands may use visual and/or audible outputs.
TicTracers are used primarily as a screening tool for checking fuse panels, determining if a wire has voltage present, locating opens in insulated wires and heating elements hidden within walls, checking power outlets and switches, and checking for properly grounded power tools and appliances. The instrument is not effective for determining if voltage is present on wires inside a metal conduit or tubing.
The advantages of the TicTracer are it's low cost, small size, simplicity and ease of use. The compliance officer can screen a lot of equipment in a short time.
The following list gives a few examples of how a compliance officer may make use of a multimeter. Those applications for which pocket size multimeters are well suited are indicated by asterisks.
** Battery Packs. Batteries should be tested under load. The easy way to do this is to use a multimeter to measure the voltage while an instrument is running. Another tool the compliance officer has for checking batteries is the Simpson Model 379. It is simply a special purpose voltmeter designed specifically to test batteries. Unfortunately it is a little bulky and it wasn't designed with NiCad battery packs in mind. Another alternative is to place a load resistor across the battery terminals and measure the voltage with the multimeter. If you have an application requiring a load resistor, you may call the Cincinnati Laboratory for assistance.
** Battery chargers. Battery chargers are one of the most common instruments used by compliance officers. This fact is often over looked. The multimeter is an excellent tool for checking a charger. However, a common error is the expectation that an open charging jack will have the same voltage reading as when it is plugged into the battery pack. Constant current chargers (the recommended type for NiCad batteries) will have a much higher open circuit voltage than they will under normal charging load. Constant potential chargers (the type frequently used for sealed lead acid batteries) will usually have an open circuit voltage closed to but slightly higher than under normal charging load. Constant potential chargers are also referred to as constant voltage chargers and trickle down chargers.
** Recorder outputs of instruments. When a recorder is to be used with an instrument and the recorder is not responding, the multimeter can be substituted for the recorder to check if proper output is present.
** Remote readout. The multimeter AC or DC voltage scales can be used as a remote readout in the absence of custom made readouts or if the regular readout has been damaged. The multimeter is connected to the instrument's analog output jack. The voltage readings can be converted to a direct reading of such parameters as PPM, flow rate or percent concentration by the use of applicable conversion (scaling) factors.
** Test leads and cables. One of the most common problems for a technician, or operator that uses electronic instrumentation is damaged cables and leads. Portable field applications aggravate the problem. CSHO's will encounter more and more situations where they must connect recorders to instruments, data loggers to instruments, power packs to instruments, chargers to power packs, microphones to instruments, remote control lines to instruments, remote sensors to instruments, etc. They will find that cables and test leads are frequently broken or shorted at the assembly point between the cable and the jack or plug. The first trouble shooting step should be to use a multimeter to measure the end to end resistance of the cables and test leads. This simple step will quickly identify open or shorted cables.
** Lamps. Your flashlight won't work. Check the resistance of the bulb and check the operating voltage of the battery.
** Fuses. This seems simple enough but it is often over looked. Glass encapsulated fuses can be inspected visually about 95% of the time but occasionally the operator can be deceived by what appears to be a good fuse. Ceramic encapsulated fuses require the use of a multimeter. A blown fuse will have an infinite resistance reading and a good fuse will indicate a short or very low resistance.
-- Leakage of AC voltage. Check for the presence of unwanted AC voltage on metal objects, such as light fixtures, metal encased equipment, conduit, etc. This AC voltage generally indicates the presence of two undesirable conditions. The first is inadequate grounding of the metal object. The second is a conductive path between the active circuit and the case or other metal component. The conductive path could result from faulty insulation, shorted components or even poor design.
-- Live circuits. Check for presence of voltage on a circuit.
-- Wiring Polarity of receptacles. Check for proper connection of "hot" and neutral conductors to an electrical outlet receptacle. The proper readings are: 120 volts between hot and neutral; 120 volts between hot and ground; and very low voltage between neutral and ground.
-- Line voltage. Check for the proper line voltage and for fluctuations in the voltage.
-- Ground connections. A ground connection is suspected as being missing. Check the resistance of the object to ground.
-- Loss of power. An AC powered instrument won't work or turn ON. Check the duplex outlet for AC voltage. If voltage is present, disconnect the plug and check the resistance across the instrument plug, including any extension cord for proper connection. If open circuit, power is not getting to the instrument or the instrument's fuse is blown.
-- Shielding. Check integrity of shielding connections.
-- Sampling pump performance test. A multimeter can be used in conjunction with sample pump calibrator kits to make more complete performance checks of pumps. The details of a simple, inexpensive test set-up is available through the Cincinnati Laboratory.
The following list gives some of the safety concerns when using electrical testers. This is not a comprehensive list that covers routine safety practices that would normally be covered in safety compliance courses or electrician courses, rather it covers some of the characteristics and practices that are peculiar to the instruments OSHA uses.
MULTIMETERS HAZARDS AND MYTHS
A myth some times heard about multimeters is that the small test leads might melt when used to test power circuits. Although no such incidence has ever been reported to the Cincinnati Laboratory, it is a persistent myth.
The voltage measurement ranges of multimeters are designed with input impedances of one megohm (1,000,000 ohms) or better. Even when measuring 1,000 volts, the current in the test leads is less than 1 milliampere (usually in the microampere range). Such current levels would not melt even very small wires.
When measuring currents, the potential could exist for melting test leads if the multimeter were a low quality, poorly designed unit. This is because the current measuring mode of a typical multimeter requires that it be connected in series with the circuit under test and that all of the circuit current be routed through the meter. To protect against exceeding the current rating of the meter, any quality multimeter has a fuse in-line with the test leads. The Fluke Model 21 used by OSHA has a maximum built in current range of 300 milliamps with a fused input. The fuse will blow long before the melting point of the test leads is reached. Using the built in current measuring functions of a multimeter requires a deliberate act to disconnect the circuit under test and to connect the meter in series with the circuit. The current measuring mode of operation isn't going to occur by accident nor is it intended that OSHA's compliance officers use the Fluke Model 21 in this fashion on AC power circuits. The danger isn't melting test leads; it's personal exposure to active power circuits. A clip-on AC current probe (covered in following text) has been provided for such applications because it is safer and easier. There is no danger of melting a test lead when using the clip-on probe.
The pocket-sized multimeters (Heath SM 2300) that compliance officers have been furnished don't have a current measuring mode so an attempt at such an application would be a severe misuse of the instrument.
Trying to use the resistance modes of a multimeter on a live circuit is a severe misuse of the instrument. Such a misuse will probably burn out some of the internal components of the meter but it's most unlikely to melt the test leads. This is the most common cause of multimeter failures. The Fluke Model 21 has built in protection for this type of misapplication. The Heath SM 2300 is such a low cost item that it would not be worth repairing after such damage.
A problem that is significant at high voltages is the integrity of the insulation covering the probe and test leads. At 3500 volts and above, current will arc across an air gap of approximately 1/32 of an inch. Some hookup wire has insulation that is this thin. Small holes or cracks in the insulation will provide the necessary arcing path. Grease, sweat or other contaminants that penetrate the holes will enhance the current or arcing path and create a shock hazard at relatively modest voltages. OSHA's compliance officers should not directly connect meters or probes to high voltages (rule of thumb is 500 volts or above) unless they are specifically trained for this and have the appropriate high voltage test probes.
When sparks fly during the use of a multimeter, it's usually because the operator has shorted the circuit under test with the tips of the test probe, for example, inadvertently touching the side of the probe to a case while measuring the power supply voltage. The test leads aren't likely to melt but the tips of the test probe will probably have some burn marks.
Another myth seems to be that the multimeter is such a versatile tool that it makes many other electrical testers redundant. Although a multimeter is, indeed, versatile, some other instruments have features that allow them to make measurements easier and safer. Consider, for example, the simple 120 VAC receptacle testers. A survey of 120 VAC receptacles that might take one hour with a receptacle tester could take all day using the multimeter. Another good example is the ECOS Model 1020 Outlet & Leakage Analyzer. Although it is possible to use a multimeter for this application, it is impractical and would cause a severe work disruption. In other cases such as the GFCI Tester or the Megohmmeter, there is no alternative procedure using a multimeter.
GROUND WIRE IMPEDANCE TESTERS (WOODHEAD GLITs & ECOS MODEL 1020:
As stated in the field service memo, the ground wire impedance tester is a specialized device for testing the impedance of the grounding wire to 120 VAC receptacles. This device achieves this by injecting a current pulse(s) on the ground wire of the circuit. The magnitude of the pulse or pulses is sufficient to cause and electrical shock under certain specific conditions. If the 120 volt receptacle under test is wired properly with a ground wire connected back to the circuit panel buss (all the way back to the point were the neutral and grounding wire are connected to a common buss), there is no risk of a shock. If, on the other hand, the grounding wire is defective, the potential does exist for a shock.
A shock could occur under the following set of circumstances:
1. As stated, the grounding wire is defective (e.g. broken). 2. The operator of the ground wire impedance tester fails to observe the wiring indicator lamps.
3. The circuit under test has other receptacles which are actively being used.
4. The equipment plugged into the circuit at the other receptacles are case grounded tools.
5. The break in the grounding wire occurs at a point closer to the panel box than either of the receptacle under test or the receptacle in use.
6. The operator of the ground wire impedance tester depress the ground wire test button and sends a current pulse down the defective ground wire.
7. The operator of a case grounded tool can receive a short duration shock. The magnitude of the shock will depend on how well his body forms a ground path.
8. Although the shock will be in the painful range it would not be lethal due to the short duration. The Woodhead Glit uses only a single shoot type pulse while the ECOS Model 1020 has a safety circuit which senses an impedance greater than 75 ohm and automatically cuts itself off after a few pulses.
Receptacle testers are very simple devices used to determine if a 15 or 20 ampere, 120 VAC electrical outlet has been wired correctly. A certain lite patterns indicates whether the ground is open or connected. The device will sense open grounding wires or high resistance grounding paths but there are certain rare conditions under which it can give false indications. The device's detection method is a simple neon bubble inseries with a resistor that connects between the hot wire and the grounding wire. If both the hot and grounding wires are connected correctly a two to three milliamp current will flow into the grounding wire and turn on the light. Sufficient current will flow even with a relately high impedance in grounding wire (up to 1500 ohms). In fact, it is possible to disconnect the grounding wire and turn on the lamp by forming a current path with your fingers connected between the neon light circuit and ground.
In short, receptacle testers are not a perfect device. There are certain fault conditions that they will not detect.
|NOTICE: This is an OSHA Archive Document, and may no longer represent OSHA Policy. It is presented here as historical content, for research and review purposes only.|
Standard Interpretations - (Archived) Table of Contents|