A great deal of attention is devoted to safe working practices relating to electrical construction, maintenance and repair work. Industry electrical publications regularly report on safety issues, including the use of the proper tools and equipment for energised and de-energised work, as well as using the correct personal protective equipment (PPE) for each workplace situation. However, electrical test instruments are given very little, if any, discussion in safety articles. Even the dangers of using the wrong test instrument or using an instrument improperly, which can have catastrophic results, are rarely mentioned.
Some of the most frequently used test instruments include non-contact voltage testers, multimeters, insulation testers and ground-resistance testers. A big issue with using non-contact or proximity devices, for example, is that to prove a circuit is de-energised it is necessary for that circuit to be tested phase-to-phase and phase-to-ground, which cannot be done using this type of tester.
When electrical safety is discussed, the subjects of shock, arc flash, and arc blast predominate in the discussions. The question is often asked: How do I identify when these hazards are present, or likely to be present, when I am using electrical test instruments on electrical circuits and equipment? This article discusses electrical hazards, along with requirements for assessing the workplace to identify electrical hazards, and also discusses personal protective equipment (PPE) associated with using test instruments.
Electricity is widely recognized as a serious workplace hazard, exposing employees to risk of electrical shock, electrocution, burns, fires, and explosions. Employees have been killed or injured in fires and explosions caused by electricity.
An additional consideration related to the electrical hazards of arc flash and arc blast is that extremely high-energy arcs can damage equipment, causing fragmented metal to fly in all directions. In atmospheres that contain explosive gases or vapours, or combustible dusts, even low-energy arcs can cause violent explosions. In these cases the electric arc may be the ignition source for a much bigger explosion and fire.
Because of the potential electrical hazards associated with the use of electrical test instruments, only qualified persons are permitted to perform tasks such as testing, troubleshooting, and voltage measuring when working within the Limited Approach Boundary of exposed energized electrical conductors or circuit parts operating at 50 volts or more, or where any other electrical hazard may exist.
Improper use of electrical test instruments can result in shock or electrocution, as well as creating an arc flash incident. This article addresses these issues, along with the requirements for selecting and using test instruments to verify the presence of voltage.
Selection of Electrical Test Instruments
Regardless of whether you are performing electrical installation work, equipment maintenance, verifying the absence of voltage for de-energized work, troubleshooting, making voltage measurements or carrying out other diagnostic work, it is imperative to collect accurate and consistent information from the tests you perform. In order to comply with electrical industry standards and regulations, there is a need to select and use the right test instruments according to the application.
When conducting voltage verification, for energized and de-energized work, the electrical worker must select the right test instruments and equipment for the work to be performed. As a minimum, these should include the following:
- Voltage indicating instrument suitable for conditions
- Correct category (I, II, III, or IV)
- Continuity test instrument
- Insulation resistance test instrument
All test instruments must be accompanied by the manufacturer’s operational instructions. The test instruments must be certified and display a label of an independent verification lab, such as UL, CSA, CE, ETL or TUV. Make sure all meters, test leads and probes have an adequate category (CAT) safety rating. Sometimes, the only thing standing between an electrical worker and an unexpected voltage surge is their meter and test leads. If you use the wrong equipment with the wrong voltage, you could be putting yourself and others at risk. So, before conducting any test, make sure your choice of instrument is correct.
Electrical standards, such as UL, ANSI, IEC, and CAN specify protection from currents at levels well above a system's rated capacity. Without this additional protection, transient overvoltages, which are becoming increasingly common, can lead to equipment failure and serious injury or death.
Minimizing such risks requires that everyone working in electrical environments has safety equipment as required. They need correctly rated gloves, eye protection, and electrical test instruments that provide appropriate protection. Having the correct electrical test and measurement instruments and using the correct procedures can improve job safety.
In connection with this, a quick review of the four category (CAT) ratings is in order:
Category I — typically covers electronic equipment. Signal level for telecommunications, electronic equipment, and low-energy equipment with transient-limiting protection. The peak impulse transient range is from 600 to 4,000 volts with a 30-ohm source.
- Protected electronic equipment
- Equipment connected to (source) circuits in which measures are taken to limit transient overvoltages to an appropriately low level
- Any high-voltage-low-energy source derived from a high-winding resistance transformer, such as the high-voltage section of a copier.
Category II — single-phase receptacle connected loads. Local level for fixed or non-fixed powered devices – everything from lighting to appliances to office equipment. Also, all outlets at more than 10 m (30 feet) from Category III sources and all outlets at more than 20 m (60 feet) from Category IV sources. The peak impulse transient range is from 600 to 6,000 volts with a 12-ohm source.
- Appliance, portable tools and other household and similar loads
- Outlet and long branch circuits
- Outlets at more than 10 metres from CAT III source
- Outlets at more than 20 metres from CAT IV source
Category III — three-phase distribution, including single-phase commercial lighting; distribution level-fixed primary feeders or branch circuits. These circuits are usually separated from Category IV (whether utility service or other high- voltage source) by a minimum of one level of transformer isolation; for example, feeders and short branch circuits, distribution branch panels and heavy appliance outlets with "short" connections to service entrance. The peak impulse transient range is from 600 to 8,000 volts with a 2-ohm source.
- Equipment in fixed installations, such as switchgear and polyphase motors
- Bus and feeders in industrial plants
- Feeders and short branch circuits, distribution panel devices
- Lighting systems in larger buildings
- Appliance outlets with short connections to service entrance
Category IV — three-phase at utility connection, any outdoor conductors or primary supply level. It will cover the highest and most dangerous level of transient overvoltage you are likely to encounter-in utility service to a facility both outside and at the service entrance, as well as the service drop from the pole to the building, the overhead line to a detached building, and the underground line to a well pump. The peak impulse transient range is from 600 to 12,000 volts with a less than 1-ohm source.
- “Origin of installations,” such as where low-voltage connection is made to utility power
- Electricity meters, primary overcurrent protection equipment
- Outside and service entrance, service drop from pole to building, run between meter and panel
- Overhead line to detached building, underground line to well pump
Use of Electrical Test Instruments
As previously stated, due to the potential electrical hazards associated with the use of electrical test instruments, only qualified persons are permitted to perform tasks such as testing, troubleshooting, and voltage measuring when working within the Limited Approach Boundary of exposed energized electrical conductors or circuit parts operating at 50 volts or more, or where any other electrical hazard may exist. Improper use of electrical test instruments can result in shock or electrocution, as well as creating an arc flash incident.
The following additional requirements apply to test instruments, equipment, and all associated test leads, cables, power cords, probes, and connectors:
- Must be rated for circuits and equipment where they are utilized
- Must be designed for the environment to which they will be exposed and for the manner in which they will be utilized
- Must be visually inspected for external defects and damage before each use
- If there is a defect or evidence of damage that might expose an employee to injury, the defective or damaged item shall be removed from service
When test instruments are used for testing the absence of voltage on conductors or circuit parts operating at 50 volts or more, the operation of the test instrument must be:
- Verified on a known voltage source before an absence of voltage test is performed
- Used to test for the absence of voltage on the de-energized conductor or circuit part, remembering that
- A zero reading might mean that no voltage is present during the testing, or
- It could mean that the instrument has failed
- Verified on a known voltage source after an absence of voltage test is performed
This verification primarily applies to conductors or circuit parts operating at 50 volts or more. However, under certain conditions (such as wet contact or immersion) even circuits operating under 50 volts can pose a shock hazard.
Only qualified persons are permitted to perform tasks such as testing, troubleshooting, and voltage measuring, due to the electrical hazards associated with energized work. All required PPE for the associated hazards must be used when performing these tasks. Test instruments must be rated for the conditions under which testing is to be performed. When selecting voltage-testing instruments, an assessment must be performed to determine the proper category (CAT) rating required, based on the highest hazard exposure.
When test instruments are used for testing the absence of voltage, for de-energized work, on conductors or circuit parts operating at 50 volts or more, the operation of the test instrument must be verified on a known voltage source before and after an absence of voltage test is performed.