Purpose
No circuit should ever be presumed dead until it has been measured using reliable, prechecked test equipment. Good safety practice and current regulatory standards require that circuits be certified de-energized by measurement as the last definitive step in the lockout- tagout procedure.
Instrument Selection
Voltage-measuring instruments should be selected for the category, voltage level, the appli- cation location, short-circuit capacity, sensitivity requirements, and the circuit loading requirements of the circuit which is to be measured.
During voltage measurement, the instrument may be subjected to voltage transients. This is especially critical in low-voltage (below 1000 V) applications. When selecting a low-voltage-measuring instrument, be certain to select the correct category of instrument as defined in IEC 61010. See Chap. 2 for a description of the categories.
Voltage Level. The instrument must be capable of withstanding the voltage of the circuit which is to be measured. Use of underrated instruments, even though the circuit is dead, is a violation of good safety practice.
Application Location. Some instruments are designed for outdoor use only and should not be used in metal-clad switchgear. Always check the manufacturer’s information and verify that the instrument is designed for the location in which it is being used.
Internal Short-Circuit Protection. Industrial-grade, safety-rated voltage-measuring instru- ments are equipped with internal fusing and/or high-resistance elements which will limit the short-circuit current in the event the instrument fails internally. Be certain the instrument being used has internal protection with an interrupting/limiting rating that is at least equal to the short-circuit capacity of the circuit that is being measured.
Sensitivity Requirements. The instrument must be capable of measuring the lowest nor- mal voltage that might be present in the circuit which is being measured. If the instrument has two ranges (high and low, for example), be certain to set it on the correct lowest range which applies to the voltage being measured.
Circuit Loading. A real voltmeter can be represented by an ideal meter in parallel with a resistance. The resistance represents the amount of loading the meter places on the circuit (Fig. 3.21). In the diagram RSrepresents that resistance of the system being measured and
RMis the internal resistance of the voltmeter. The voltmeter will read a voltage given by the formula:
(3.1) If the meter internal resistance is high compared to system resistance, the voltage across the meter will be very close to the actual system voltage. If the meter resistance is very low compared to the system resistance, the meter will read a lower voltage than actually exists because most of the voltage drop will be across the system resistance instead of the meter resistance. In this second situation, the meter is “loading” the circuit.
If a very low resistance meter is used, the meter may not read a potentially lethal static or inductively coupled voltage. This situation is unacceptable since an inductively coupled or static voltage can be lethal. The meter that is selected for a safety-related voltage mea- surement must have an internal resistance that is high enough to avoid this problem.
Instrument Condition
Before each use, an instrument must be closely inspected to ensure it is in proper working order and that insulation systems have not been damaged. The case physical condition, probe exposure, lead insulation, fusing, and the operability of the instrument should be verified.
Case Physical Condition. The case must be free of breaks, cracks, or other damage that could create a safety hazard or misoperation. Broken instruments should be taken out of service until they can be repaired or replaced.
Probe Exposure. Modern instrument probes have spring-loaded sleeves which cover the probe until forced back. Check to make certain that only the minimum amount of probe required to do the job is exposed.
V V R
R R
M M
M S
= × +
SAFETY PROCEDURES AND METHODS 3.33
V VM
RS
RM
Circuit being measured Voltage measuring instrument
3.34 CHAPTER THREE
Lead Insulation Quality. Carefully inspect the lead insulation to make certain it is not damaged in any way.
Fusing. Accessible fuses should be checked to be certain that they are correctly installed and have not been replaced by incorrect units.
Operability. Before each usage (at the beginning of each shift, for example), the instru- ment should be checked to make certain that it is operable. Do not substitute this check for the instrument checks required in the three-step process.
Three-Step Measurement Process
Step 1—Test the Instrument. Immediately before each measurement, the instrument should be checked on a source which is known to be hot. This step confirms that the instru- ment is working before the actual circuit verification is made.
The preferred method is to use an actual power system conductor of the same voltage as that being verified. Finding such a circuit is easier when low-voltage measurements are being made; however, even then a hot circuit may not be available. Some manufacturers supply a device which creates a voltage that is sufficient for testing the instrument.
Some instruments intended for measurement of medium voltages have a low-voltage switch setting on them which allows them to be checked on low voltage. This is not a preferred method since a problem with the switch could give erroheous, dangerous false readings.
Step 2—Measure the Circuit Being Verified. After the instrument is tested, the worker then measures the circuit being verified to make certain it is de-energized. The actual wires that should be measured and the techniques to be used are discussed later in this chapter.
Step 3—Retest the Instrument. After the circuit has been verified, the instrument should be rechecked as it was in step 1. This ensures that the instrument was operable both before and after the measurement, thus affirming that a zero measurement is zero and not caused by an inoperable instrument.
What to Measure
As a general rule of thumb, all normally energized conductors should be measured to ground and to each other. The readings to ground should be made whether the system is grounded or not. Note that all readings should be made as close to the point of exposure as possible.
The procedures given in the following paragraphs will apply only to contact type instru- ments. Proximity instruments measure the electrostatic field set up around an energized conductor rather than the actual voltage between two conductors. If the proximity indica- tor shows an unexpected voltage, the circuit should be rechecked, possibly with a contact tester.
Single-Phase Systems. The hot wire of single-phase systems should be measured both again neutral and ground. Figure 3.22 shows the points which should be measured.
Two-Phase Systems. A voltage measurement should be taken between the two hot phases, between each hot phase and neutral (one at a time), and between each hot phase and ground. Figure 3.23 shows the measurement locations for a typical two-phase system.
Three-Phase Systems. Measure between each of the hot wires, two at a time, between each hot wire and neutral, and between each hot wire and ground. Figure 3.24 shows the measurement locations for a typical three-phase system.
Using Voltage Transformer Secondaries for Voltage Measurements. Many medium- voltage power systems are equipped with step-down voltage transformers which are used for metering or telemetering purposes. Because of the lower voltage on the secondary, some workers wish to measure the secondary winding voltage to verify that the primaries are de- energized. Such transformers may be used for safety-related voltage measurements under the following conditions:
1. The transformers can be visually traced and are known to be connected to the system
where exposure will occur.
2. The transformers are located close to the part of the system where exposure will occur.
SAFETY PROCEDURES AND METHODS 3.35
Hot wire (H)
Neutral wire (N)
Ground wire (G)
Measurement to be made at location where exposure
will occur. Measurements: 1. H to N
2. H to G
FIGURE 3.22 Measurement points for a single-phase system (one hot wire).
1. H1 – H2 2. H1 – N 3. H2 – N 4. H1 – G 5. H2 – G Hot wire (H1) Neutral or ground (N or G) Hot wire (H2) Measurement to be made at location where exposure
will occur. Measurements:
3. The transformer secondaries must be measured both before and after the circuit is de-
energized. This verifies that the transformer fuses are not blown, resulting in an erro- neous zero voltage reading.
4. A safety ground is applied to the primary circuit of the transformer after the measure-
ment but before personnel contact occurs.
5. All other safety-related techniques should be used as described in the other parts of this
chapter.
Caution: This procedure is considered a secondary, nonpreferred method. Do not use this approach unless absolutely necessary.
Using Panel Voltmeters for Voltage Measurements. Panel voltmeters may be used as a general indication of system energization; however, they should not be used for safety- related voltage measurements.
How to Measure
Preparation. Table 3.11 identifies the steps which should precede the actual measure- ment procedure. The area should be cleared of unnecessary personnel. This will prevent their exposure to arc or blast in the event that a problem occurs. The person making the
3.36 CHAPTER THREE
A phase (A) B phase (B)
Neutral ground (N or G)
Measurement to be made at location where exposure
will occur. Measurements: C phase (C) 1. A – B 2. B – C 3. C – A 4. A – N 5. B – N 6. C – N 7. A – G 8. B – G 9. C – G
FIGURE 3.24 Measurement points for a three-phase system.
• Clear area of unnecessary personnel. • Wear appropriate safety equipment. • Expose conductors to be measured. • Position measuring instrument.
SAFETY PROCEDURES AND METHODS 3.37
measurement should wear and use appropriate safety equipment. (Safety equipment infor- mation will be covered later in this chapter.)
After the safety equipment is on, the panels, doors, or other access means should be opened to expose the conductors that are to be measured. The measuring instrument is then carefully positioned. When making medium-voltage measurements with hot sticks, be certain that the hot stick is not contacting your body. The instrument should be securely positioned so that it does not fall to the floor if the leads are inadvertently overextended.
Safety Equipment. Table 3.12 lists the minimum recommended safety equipment to be worn when making low-voltage measurements. Table 3.13 lists equipment for medium- voltage measurements. The flash suit is identified as optional for outdoor, open-air mea- surements. Although many line personnel do not wear flash suits when performing overhead work, the use of these uniforms is strongly recommended.
Measurement. After all preparations are made and the safety equipment is put on, the measuring instrument should be applied to the conductors. If a measurement to ground is being made, one lead should be connected to the ground first and then the phase connec- tion made. When measuring between two hot wires, the order of connection is unimpor- tant. If a contact instrument is being used, each lead should be carefully placed on the appropriate conductor. The meter or readout is then observed to see if the circuit is hot. If a proximity instrument is being used, it should be moved gradually toward the conductor until it indicates or until the conductor is touched.