CHRIS HALLIDAY: Test lamps
Many electricians have relegated test lamps to the museum. Should the remaining old-school sparkies update or are the new mob just showing off expensive toys? Chris Halliday tests the topic.
The AVO 8 was a high-quality test instrument in common use when I started in the industry in the mid-1970s. Test lamps have been around even longer.
This topic emerged when I started outlawing the use of test lamps by line workers at my old employer in the early 2000s. I gave a talk on the issue at the Utilities National Conference in Canberra in 2003 – with a varied response from attendees. Later I found that only some depots were free of test lamps.
The topic resurfaced recently with a client looking to improve testing practices. I thought I’d persuaded him to ditch test lamps, but the discussion persists.
So let’s look at the advantages, the disadvantages and when each method might come into its own.
Test lamps are relatively cheap and provide a quick indication of supply volts.
No calibration is required, thus saving considerable annual calibration costs. Replacement globes are also cheap.
Testing the tester is always important, especially when proving ‘dead’. A continuity tester for checking that globe filaments are intact is readily available, and again fairly cheap.
On the flip side, globes are easily broken by vibration and dropping, and they often blow when voltage is applied. So, when you most need them they are useless.
Test lamps don’t light up until about 90V, depending on the available light and the type of globe. It should be remembered that any current above 50V can be lethal – you could prove ‘dead’ using test lamps and end up dead yourself.
(I’ve never heard of this happening, and there would be more sparkies alive today if they had actually used a test lamp to prove ‘dead’.)
Other than showing ‘on’ or ‘off’, test lamps don’t give much indication of voltage levels – unless you count extremely low (fails to light) or extremely high (blows the globes) – and they are of limited use when fault finding.
One of my old employers used lamps for a ‘final test’ when connecting services to ensure the correct polarity. The sparkies would test from the neutral link to a remote earth reel: the test lamps didn’t light up for a normal connection and did light up to show reverse polarity.
But, depending on soil resistivity, there could be a nasty voltage on the top of the remote earth reel/stake. There were numerous stories of dogs getting bad shocks from sniffing the reel/stake.
Admittedly, lamps can be useful for detecting high impedance faults, whereas a multimeter may show full line voltage when clearly there is a fault (see The shocking truth, part 2, spring issue 2013).
Test lamps can be used as a final check to ensure correct circuit connections and that there are no circuit intermixes, but any load could do this for you. Simply test from active to earth at a socket outlet with your lamps and the correct safety switch should trip.
A more modern and thorough approach would be to use a loop tester on the ‘no trip’ range for each socket outlet; and then on the last outlet on the circuit step up to the high-current, loop-test range.
Multimeters are versatile instruments with multiple ranges, thereby adding a degree of complexity, but we are talking about tradies and their main tool of trade.
Modern digital multimeters are very robust and must generally be capable of withstanding a 1m drop on a corner of the device onto a solid surface.
Multimeters provide an indication of voltage level, current, ohms, etc, making them extremely useful.
Multimeter results can be used when making informed decisions on what needs to occur.
Let’s say you are about to install an additional socket outlet in a kitchen and your testing reveals a reading of 260V. You could check neutral integrity and, if it was OK, contact the network company to have the voltage lowered.
Very high or very low voltage can cause a fire or indicate a faulty neutral. People’s lives may be at risk, so multimeters come into their own in such situations – compared with the on/off indication of test lamps.
Multimeters are more expensive, and there will be calibration costs (see the spring edition mentioned on why you should calibrate test instruments). Some multimeter ranges, depending on the model, are unlikely to be used by typical electricians, so some would be paying for features they didn’t need. Therefore choosing the right instrument for the job is important.
A multimeter battery may go flat, but the operator generally has ample warning to replace the battery. A word of warning though: change the battery as soon as possible, as the meter will start giving dodgy readings as the battery runs down.
A multimeter with an inbuilt tong or add-on tong adds another degree of versatility, as AC and DC can be measured by the tong.
Testers are available for checking the operation of a multimeter on the voltage range. These testers are called proving units and generate about 230V. As with the test-lamp tester, you should prove your multimeter before proving ‘dead’ – and again after a no-go result.
Multimeters can show full voltage for high-impedance faults or inductive or capacitive coupling, whereas test lamps will dissipate the leakage current.
Some meters have a LoZ range, or you can use a low-impedance resistance plugged into the terminals of the multimeter, eg: CIC shock investigation filters designed to AS/NZS60990.
Why change to a multimeter?
Litigation is rampant in Australia, so electricians need to ensure that a professional job is carried out at all times.
Imagine if you were the last professional at an electrical installation and your test lamps showed everything to be OK but the house burned down that night from overvoltage. Could you sleep at nights? How would your insurance company view this?
There’s no guarantee that the network company is supplying the right voltage, so you should check that your client is receiving supply within Standard at all times (see When voltage varies, winter issue, 2012).
Low and high voltages are an indication of poor neutral integrity. Water taps or other equipotentially bonded or earthed metalwork may be ‘live’, and you wouldn’t want to leave your customers exposed to risk.
Test lamps may not help identify such problems, but multimeters will. Electricians need proper results for making informed decisions, and a multimeter will do this for you.
Old-school tradies should retire their test lamps and use multimeters as a matter of course. Understanding the test instrument will allow better interpretation of test results. Your clients, and your business, will feel the benefits.
Disclaimer: This article is written on behalf of the Institute of Electrical Inspectors, although the views expressed are not necessarily those of the institute. Any information in this article is given in good faith and is not all-encompassing. Electricians should make their own decisions based on legislative requirements, Standards, codes of practice, risk assessment, knowledge, etc. Otherwise seek further assistance.