Safety Voltage Measurement
Safety voltage measurement actually involves measuring for zero voltage. That is, a safety measurement is made to verify that the system has been de-energized and that no voltage is present. (See Chap. 3 for more details.) Because of this, the instruments that are used for
(b)
FIGURE 2.56 Locking devices. (Courtesy Ideal Industries, Inc.) (a)
safety voltage measurement need only be accurate enough to determine whether or not the nominal system voltage is present at the point of worker exposure.
The instruments discussed in the following sections are intended primarily for safety voltage measurements. Some of them are quite accurate. Contact the individual manufac- turers for more specific information.
For various reasons, some authorities prefer a proximity tester over a contact tester, while others prefer a contact tester over a proximity tester. Whichever type is selected, it must be capable of indicating the presence of dangerous voltages and discriminating between nuisance static voltages and actual hazardous voltage source. See Chap. 3 for more information.
Proximity Testers
Proximity testers do not require actual metal- to-metal contact to measure the voltage, or lack of voltage, in a given part of the system. They rely on the electrostatic field established by the electric potential to indicate the pres- ence of voltage. Proximity testers will indi- cate voltage levels through insulation. They may not provide accurate results when cable is shielded.
Proximity testers are not accurate and do not indicate the actual level of the voltage that is present. Rather they indicate the presence of voltage by the illumination of a light and/or the sounding of a buzzer. Figures 2.58 through 2.60 are three different types of prox- imity detectors for various voltage levels.
ELECTRICAL SAFETY EQUIPMENT 2.55
FIGURE 2.57 Typical application of locks, tags, and multiple-lock devices. (Courtesy Ideal Industries, Inc.)
FIGURE 2.58 “AC Sensor” proximity voltage sensor for use on circuits up to 600 V alternating cur- rent. (Courtesy Santronics, Inc., Sanford, NC.)
2.56 CHAPTER TWO
Figure 2.58 shows a simple neon light proximity tester. The end of the unit is plastic and sized to fit into a standard 120-V duplex receptacle. It requires two AAA flashlight cells to operate. When placed in proximity to an energized circuit, the red neon, located in the white plastic tip, glows.
Figure 2.59 shows a proximity tester that uses a combination of audio and visual indi- cators. When turned on, the unit emits a slow beeping sound which is synchronized to a small flashing light. If the unit is placed close to an energized circuit, the number of beeps per second increases. The light flashes increase in frequency along with the beeps. The higher the voltage and/or the closer the voltage source, the faster the beeps. When the voltage is very high, the beeping sound turns into a steady tone. This unit has a three-posi- tion switch. One position turns the unit off, and the two others provide low- and high- voltage operation, respectively. If high volt- ages are being measured, the unit should be attached to a hot stick. The manufacturer sup- plies a hot stick for that purpose.
Figure 2.60 shows a proximity tester that is similar in some ways to the one shown in Fig. 2.59. It has both a light and a tone; how- ever, the light and tone are steady when a voltage is sensed above the unit’s opera- tional threshold. The unit in Fig. 2.60 also has a multiposition switch which has an OFF position, a 240-V test position, a battery test position, and multiple voltage level posi- tions from 4200 V up to the unit’s maxi- mum. This unit is shown attached to a hot stick in Fig. 2.46. It should not be used to measure voltages without the use of a hot stick and/or appropriate rubber insulating equipment.
Contact Testers
Some personnel prefer the use of testers which make actual metal-to-metal contact with the circuit being energized. Such instru- ments are called contact testers. Contact testers may be simple indicators, but more often they are equipped with an analog or digital meter which indicates actual voltage level. Figures 2.61 through 2.64 illustrate
FIGURE 2.59 “TIC” tracer proximity voltage sensor for use on circuits up to 35,000 V. (Courtesy TIF Electronics.)
FIGURE 2.60 Audiovisual proximity voltage tester for use in circuits from 240 V to 500 kV. (Courtesy W.H. Salisbury and Co.)
various styles of contact testers that may be used for safety-related measurements.
Figure 2.61 shows one of the more popu- lar models used for voltages up to 600 V alternating current or direct current. This unit is a solenoid type of instrument. That is, a spring-loaded solenoid plunger is connected to an indicator which aligns with a voltage scale. The distance that the plunger travels is proportional to the voltage level of the mea- sured circuit. The voltage scale is read in volts. The instrument shown in Fig. 2.61 also indicates continuity and low voltage. It switches automatically between those func- tions. Note that this style of unit is one of the most popular styles in use for low-voltage measurements. Because of their solenoid mechanism, these styles are subject to small arcs when the contact is made with the mea- sured circuit. This problem can be mitigated by using test leads equipped with fused resis- tors. Such resistors not only eliminate the arcing problem but also open in the event that an internal short circuit occurs in the meter.
Figure 2.62 shows a modern, digital read- out safety voltmeter. This instrument is suit- able for circuits up to 1000 V and is tested to 2300 V. It has an inherently high impedance; therefore, it is not prone to arcing when the leads make contact. The meter should only be used for voltage measurements. It has no con- tinuity or ammeter scales.
Figure 2.63 shows a typical digital multi- meter. These instruments are in common use by virtually all electricians and electrical and electronic technicians. Such instruments often have voltage ranges well above 1000 V; however, use of them in power circuits with voltages above 600 V is not recommended. Care should be taken when using this style of instrument in an electric power circuit.
The instrument shown in Fig. 2.64 is called a phasing tester because it is often used to phase two circuits—that is, to check that A phase in circuit 1 is the same as A phase in cir- cuit 2 and so on. Such units can also be used for safety-related voltage measurements. The instrument is composed of two high-resistance elements in series with an analog instrument. The resistances are selected in such a way that the meter tracks accurately up to 16 kV. To extend the range of the instrument up to 48 or 80 kV, extension resistors can be added.
Selecting Voltage-Measuring Instruments
Voltage-measuring instruments must be selected based on a variety of criteria. The fol- lowing sections describe each of the steps that should be used in the selection of voltage- measuring instruments.
ELECTRICAL SAFETY EQUIPMENT 2.57
FIGURE 2.61 Safety voltage and continuity tester. (Courtesy Ideal Industries, Inc.)
2.58 CHAPTER TWO
Voltage Level. The instrument used must have a voltage capability at least equal to the voltage of the circuit to be measured. Make certain that the manufacturer certi- fies the instrument for use at that level.
Application Location. Some instruments are designed for use solely on overhead lines or solely in metal-clad switchgear. Make certain that the manufacturer certifies the instrument for the application in which it will be used.
Internal Short Circuit Protection. If the measuring instrument should fail internally, it must not cause a short circuit to appear at the measuring probes. Instruments with resistance leads and/or internal fuses should be employed.
Sensitivity Requirements. The instru- ment must be capable of reading the lowest voltage which can be present. This is from all sources such as backfeed as well as nor- mal voltage supply.
(a) (b)
FIGURE 2.62 Digital readout contact-type safety voltmeter. (Courtesy Tegam, Inc.)
FIGURE 2.63 Digital readout multimeter. (Courtesy Fluke.)
Circuit Loading. The instrument must be capable of measuring voltages that are inductively or capacitively coupled to the circuit. Therefore, it must have a high enough circuit impedance so that it does not load the circuit and reduce the system voltage to apparently safe levels.
Instrument Condition
Before performing voltage measurements, the measuring instrument must be carefully inspected to ensure that it is in good mechanical and electrical condition.
Case Physical Condition. The case and other mechanical assemblies of the instrument must be in good physical condition and not cracked, broken, or otherwise damaged. Any instrument with a broken case should be taken out of service and repaired or replaced.
Probe Exposure. Only the minimum amount of lead should be exposed on contact- type instruments. This minimizes the chance of accidently causing a short circuit when the lead contacts more than one conductor at a time. Low-voltage instruments such as those shown in Figs. 2.61 and 2.62 have spring- loaded plastic sleeves which cover the entire probe until pushed by the pressure during the measurement. These types of probes should always be used for measurement of voltages in power systems.
Lead Insulation Quality. The lead insula- tion should be closely inspected. If the insu- lation is frayed, scored, or otherwise damaged, the leads should be replaced before the instrument is used to measure voltage.
Fusing. If the instrument you are using is fused, the fuse should be checked to make certain that it is of the right size and capa- bility. Instruments used to measure power system voltages should be equipped with high interrupting capacity fuses which will safely interrupt 200,000 A at rated voltage.
Operability. Make certain that the instru- ment is operable. If it uses batteries, they should be checked before the instrument is placed in service. Check the instrument on a known hot source before it is taken into the field. If it does not work, take it out of service until it can be repaired or replaced.
ELECTRICAL SAFETY EQUIPMENT 2.59
(a)
FIGURE 2.64 Phase tester extension handles. (Courtesy AB Chance Corp.)
2.60 CHAPTER TWO
Low Voltage Voltmeter Safety Standards
Background. IEC 61010 is a standard that has been developed to define design and usage safety requirements for low voltage (less than 1000 V AC) meters. The standard applies pri- marily to contact type instruments and establishes requirements for meter categories based on the meter’s ability to withstand voltage surges such as those experienced in a modern low voltage power system.
Categories. Table 2.24 shows the four categories as defined in the standard. For industrial applications the minimum category used should be category III. If the work to be performed is in the incoming service, incoming substation, or other electrical systems close to the util- ity system, Category IV should be selected.
It is also important to note that many meters are rated for 600 V while others are rated at 1000 V. The 1000 V rated meters are always the preferred choice from a safety stand- point. In many applications, a Category III meter rated at 1000 V is superior in surge with- stand to a Category IV rated at 600 V.
Three-Step Voltage Measurement Process
Since a safety-related voltage measurement is normally made to make certain that the cir- cuit is dead, the measuring instrument must be checked both before and after the actual
(b)
circuit is read. This ensures that a zero reading is, in fact, zero and not caused by a non- functional instrument. Note that this before-and-after check should be made in addition to the check that is given when the instrument is placed into service from the tool room or sup- ply cabinet. This before-and-after measurement process is called the three-step process.
Step 1—Test the Instrument Before the Measurement. The measuring instrument should be applied to a source which is known to be hot. Ideally this source would be an actual power system circuit; however, this is not always possible, because a hot source is not always readily available— especially at medium voltages and higher.
Because of this problem manufacturers often have alternative means to check their instruments in the field. Some provide low- voltage positions on their instruments. Figure 2.65 shows the switch settings for the instrument of Fig. 2.60. This instru- ment may be verified in three different ways:
1. With the switch in the TEST-240V posi- tion, place the instrument head close to a live circuit in excess of 110 V.
ELECTRICAL SAFETY EQUIPMENT 2.61
FIGURE 2.65 Selector switch positions for the meter shown in Fig. 2.60. (Courtesy W.H. Salisbury and Co.)
TABLE 2.24 Summary of IEC Meter Categories Overvoltage
category Location of usage Examples
Category I Electronics and other types of cord • Any cord connected equipment where connected equipment locations in the supply or the equipment itself
has built-in transient suppression capabilities
• In equipment that is inherently low energy
Category II Single phase receptacle connected • Appliances
loads • Portable tools
• Long branch circuits Category III Three-phase or single-phase • Indoor lighting circuits
distribution systems which are • Bus and feeders in industrial facilities isolated from outdoor electric • Motors, switchgear, and other such utility supplies by transformers, industrial equipment
distance or other specific types of surge protection
Category IV Facilities with direct paths to • Outdoor substation facilities outdoor utility circuits and feeders • Electric meter equipment connected
directly to the utility • Service entrance equipment • Overhead lines to isolated facilities
2. With the switch in the TEST-240Vposition, rub the instrument head on cloth or clothing to obtain a static charge. The unit should indicate periodically.
3. Set the switch to the 35-KV overhead position and place the head close to a spark plug
of a running engine.
After the instrument is verified, the BATTERYposition can be used to verify the battery supply circuitry and the battery condition.
Figure 2.66(b) shows a test device and setup used to verify the type of instrument shown in Fig. 2.64. One lead from the test device (Fig. 2.66(a)) is plugged into a special jack mounted on the instrument. The other end is clipped to each of the measuring probes, one at a time. The reading on the instrument meter is used to determine the operability of the test instrument.
Of course, the best way to test an instrument is to check it on a known hot source using the scale position that will be used for the actual measurement.
Step 2—Measure the Circuit Being Verified. The instrument should then be used to actu- ally verify the presence or absence of voltage in the circuit. See Chap. 3 for a detailed description of how to make such measurements.
Step 3—Retest the Instrument. Retest the instrument in the same way on the same hot source as was used in step 1. Table 2.25 summarizes the voltage-measuring steps discussed in this section. Because of the extreme importance of voltage measurement, this procedure is repeated in more detail in Chap. 3.
General Considerations for Low-Voltage Measuring Instruments
Most general-purpose multimeters are not designed for power system use. This is certainly true of many of the small units that are available from some major retail companies. Table 2.26 lists specific requirements for test instruments used in power system voltage measurement.
2.62 CHAPTER TWO
(a) (b)
ELECTRICAL SAFETY EQUIPMENT 2.63