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Then ensure that the leads and the probes are up to the job safety-wise, and look for a substantial finger guard so you can’t accidentally touch a live conductor. Also, you’ll want to make sure the indicators will give you a clear and immediate indication of the voltage level range. We’d also suggest you look for one in a tough rugged case, as we can all be clumsy at times. A distinctive colour would also be useful, too, so you’ll easily find it in the most disorganised of toolboxes. Test lamps are another means of voltage indication, but older ones are often criticised because relying only on the light intensity to indicate the level of voltage is very subjective. The level of light indication will always be relative to the levels of ambient light. For example, a lamp that will look very bright in the subdued light of an under stairs cupboard, will look much less intense in a brightly lit kitchen. These differences will be even greater with CAT IV devices when working outside in failing light, compared with bright sunshine. It is a requirement that voltage bands are clearly labelled.
Relying on light intensity only is not acceptable. So look for modern lamps that have different bands of illumination to indicate differing voltage levels. In this way, you can access the voltage level accurately, and it will also allow clear identification of potentially lethal voltages above 50V, to enable the user to distinguish between 110V, 230V and phase to phase voltages in a three-phase system. It is also worth checking that the indication is clear, preferably with a 360° viewing angle, to make it easy to use when working in a tight corner. As with the voltage testers, make sure the leads and probes meet required standards, and look for models with an additional rotating right-angled probe, as this will greatly improve the device’s ease of use.
It would also be useful if the test lamps have the ability to draw a high current, so you can differentiate between phantom voltages and hazardous persistent voltages. Many indicators and lamps will falsely give a live indication as the lamp discharges stored energy on the circuit. Lamps that draw a high current rapidly dissipate this and correctly demonstrate the circuit is dead.
What about highly-publicised contactless devices? There is also a trend towards the use of low cost non-contact voltage testers to safely detect voltages without physical conductor contact. But these non-contact voltage testers, although very useful, are not recommended for proving that installations are dead. This is because they don’t make physical contact with the circuit – there is always a possibility of an inaccurate reading. They are far better for proving live. Most models available are similar in size to a highlighter or marker pen and can check for the presence of an AC voltage in wall sockets, cables, circuit breakers and conduits, with the tip illuminating when AC volts are detected. Some models have a built-in self check facility and can
also detect the presence and polarity of magnetic fields produced by indicators, relays, solenoids and transformers. Whatever choice you make in ensuring you do have a dead circuit, it is vital that the tester you are using is fully functional. Nothing is more dangerous than trusting a defective voltage indicator, so using a proving device before and after testing is essential.
May 2012 ECA Today 27 The
consequences of working on a live circuit, as opposed to a dead circuit, can be tragically different
Is a proving device really necessary? It’s all very well to lock off a system circuit and ensure no live source exists by means of a voltage indicator, but how do you know that your voltage indicator is working correctly? It is widely argued that it is just as effective to successively test your voltage indicator on a known live supply, the one you want to prove dead, and then again on the known live supply to prove a circuit is dead – which is all very well in theory, but the reality could present you with a whole set of problems.
What happens when you’re working on a block of flats and have to knock next door to check a live source? Even if the resident is in, will they be willing to let a complete stranger come in to start ‘messing around’ with their electrics? You run the risk of wasting time and potentially causing anxiety with the neighbours. And a block of flats is just one potential problem. What about if you are somewhere remote or stuck down a hole? There isn’t much scope to use the alternative source approach there. And it’s not just your safety at stake, what about the legal implications? The law takes this matter very seriously, and employers who fail to provide electricians with proving devices do so at their own risk. An engineering company was recently fined £300,000 following the death of one if its employees, who was electrocuted after coming into contact with a live conductor in the cable on which he was working. The employee had not been provided with the necessary test equipment to prove the cable was dead, or the means to securely isolate the circuit.
The only way to offer complete peace of mind is to check your tester with a proving device before and after you’ve proved the circuit to be dead. Proving devices enable you to quickly and conveniently verify that your voltage testers and detectors are working correctly on site. This then allows you to confidently prove dead.
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