Quite worn out after the second day of my training course at Able Skills in Dartford. Not sure that I can properly relate all that I learnt today because much of out was practical work and I didn’t feel like saying, “slow down a bit, I’d like to film that”. Today we concentrated on testing.
The tests fell into two categories: the dead tests and the live tests. Dead tests have to be performed first. Then everything was switched on for the live tests. I don’t mind admitting to feeling a slight tingle at the point of turning everything on that was just nervous anticipation rather than my circuit malfunctioning.
The first dead test is the CPC CONTINUITY TEST. Obviously it’s crucial to check the basic safety of the circuit first! CPC, as I reported yesterday, is the new acronym for “earth”. The test checks whether the CPC wire is continuous throughout the circuit. For it to do its job properly, there cannot be any breaks or gaps in it. The only way of proving that it is continuous is to measure resistance on it. To do that, it has to be connected (with a connector block inside the circuit board but without anything being connected to the circuit breakers at the board) to the line wire. To use the technical jargon, R1 (line) and R2 (CPC) are joined. Next the multi-function tester is set to measure resistance. The green and red contacts are put together and the reset button pressed to return the readout to zero. That removes any measure of resistance in the tester cables, which might otherwise skew the result. Then the red contact is put on the switch line (so as to ensure the switch part of the circuit is included in the test) and the green contact is put on the CPC. Both these points of contact were inside my ceiling rose. The readout will give a figure of less than the maximum. When the switch is thrown, the circuit created between R1 and R2 is broken and the readout will go to its maximum. We are using the AVO Megger 1552. Its maximum readout for resistance says “>999.99″. After some initial confusion as to the correct points of contact to test inside each of the roses I made yesterday, I was relieved when the test was successful. The higher of the two figures recorded has to be written on the test form. It does not matter what the figures are so long as they are beneath the maximum because that shows that current is flowing all the way along the CPC wire. In other words, it is continuous! In effect, we use line (R1) as a test cable.
For reasons that we were told would be explained later, the next test we had to be sure to complete would be the third test in the running order. This is the POLARITY TEST. Polarity is tested as part of the CPC CONTINUITY TEST. Polarity is correct when all single pole devices are in the line conductor. I just realised that when I was using the word “wire”, I should have been saying “conductor”. Having insufficient time to reword what I’ve just written, I’ll use the jargon correctly from this point on. Fuses, switches and mini circuit breakers (MCBs) are all single pole devices. They cannot be in the neutral conductor (RN) because if they were, it would be impossible to turn off the loads on the circuit. Lamps would be permanently live.
Cutting back a CPC is bad practice. Leaving it long allows switches to be changed in the future. For example, someone might want to add dimmer switches years later.
A line to CPC fault is called an Earth Fault. A line to neutral fault is called a Short Circuit. These faults are caused by overcurrents.
The conductors entering a residential house in England & Wales are: line, at 230v, and neutral & earth, at 0v.
The fuse will blow under fault conditions. In normal conditions, the resistance of a load will prevent a short circuit across an appliance.
Another type of fault is an overload. This occurs when a householder has connected too high a load across a circuit. For example, a leaky water tank might give rise to the wild notion that out would be a great idea to swiftly dry out the suddenly wet plaster by connecting six 3kW heaters to one ring circuit. 6 x 3,000W = 18,000W/230V = 78A. This won’t harm the conductors in the ring circuit because a fuse or circuit breaker will be protecting it. There is no actual fault in the ring circuit.
50-80mA (50/1000 – 80/1000 A) will cause enough damage to the human heart to be fatal. The heart is just a pump which works by dint of electrical impulses. These impulses are very low. Any interference from another electrical source in that range Is likely to cause serious harm.
Fuses are intended and designed to save a circuit, not a human life. To save life, we use a residual current device (RCD). It checks the current flowing out of the device against the current flowing back. If the current flowing back is different from the outgoing current (it would be lower), then must mean that there is a fault. If that difference reaches 30mA, within a few thousandths of a second an RCD will shut down the power. This saves lives.
The downside of an RCD is that it does not protect against an overcurrent. Therefore an RCD is used in conjunction with a circuit breaker. In other words, there are different safety mechanisms for different jobs. An RCD can and should be tested by pushing its test button. That should throw its switch.
Every domestic circuit which has been worked on in any way must be left with 30mA RCD protection. Unprotected circuits can be left alone so long as no work has been done on them – they were not installed, modified or extended.
The second downside to an RCD is nuisance tripping. Heating elements are famous for this.
An RCBO is a device which comprises an MCB and an RCD. Previously, the argument against RCBOs was cost. Now, with some shopping around, they can be got for about £20 including VAT. They used to be £50 each, a price which soon mounts up when lots of circuits were being worked on.
The second test in the list but the third one we learnt today is the INSULATION RESISTANCE TEST. In this procedure, the multi-function tester injects 500V into the system. This is not dangerous because the current is only 1-2mA, which is non fatal. If you touch this circuit when live by reason of the test, it will hurt and make your arm fly around like Harold Lloyd on top of a big drop but it won’t kill you. The three pairs of conductors each have to be tested: (i) Line and CPC; (ii) Neutral and CPC; & (iii) Line and Neutral. The order in which each pair is tested is irrelevant. The minimum acceptable readout is 1,000,000 Ohms (still no idea if omega symbol exists on my swype keyboard). If the resistance is lower than that, it means that two conductors are in contact with one another. They should not be touching! The test sheet has two columns to deal with this test. The first is marked “Line/Live” – that is for the line and neutral test. The second column is marked “Live/Earth” – that is for two tests: the line & CPC test and the neutral & CPC test. All loads have to be removed for the insulation resistance test. If they are not, the test will give a low reading which is unsatisfactory. Also, some equipment can be damaged by this test, for example anything with a silicon chip in it. The test is only for the cables. Sockets are not loads. Switches have to be closed to test that part of the circuit. Two crocodile clips are used so that there is no risk of touching the connectors (which I previously and perhaps erroneously called the multi-function tester’s ‘contacts’).
These first two tests (the CPC CONTINUITY TEST and the INSULATION RESISTANCE TEST) will identify 99% of faults. If there is no RCD, the insulation resistance test is the only way to test the insulation.
Don’t forget, no matter what you’re told, to thoroughly check everywhere (including outside) for forgotten loads. For example, security lights. Take out all lamps and close all switches.
Incidentally, prior to battery driven testers, the insulation resistance test involved hand cranking to generate 500V!
Second-hand multi-function testers have to go through a calibration test, which costs £50. Never buy a tester without an intact serial number. Apart from meaning that it has been stolen, the serial number is required for calibration. (DI-LOG 9083P is £200 cheaper than the testers used by Able Skills – the AVO Megger 1552, now discontinued. It’s been replaced by three models: the 1710, which is £460 plus vat; the 1720, at £660 plus vat; and the 1730, at £840 plus vat.) Wholesalers have calibration days. The tester must be calibrated once a year. You don’t necessarily have to return it to the manufacturers to get your calibration certificate. There’s even a mobile calibrator – google him to find out when he’s in your area.