by ericmark »
Wed Dec 28, 2011 8:29 pm
The requirement for an earth is dependent on the design of the appliance. Manufacturers are allowed to type test and by printing on the item the double square symbol declare to all that an earth is not required. This does from time to time raise problems for guys inspecting and testing as if we can't find the double square symbol we have to fail items which we are reasonable sure don't require an earth but are unable to prove it.
In today's home where all sockets under 20A are protected by an RCD there is not really a problem. Even without an earth wire it is unlikely that a shock large enough to cause irreversible damage will be experienced. However there are still homes wired to yesterdays standards.
As a result we have in a lot of cases belt and bracer's the one that comes to mind is caravans where all caravans need to have a 30ma RCD but also all caravan sites also need to have a 30ma RCD clearly there is no need for both and since the cable between site and caravan needs protecting only the RCD on the site is really required. But the rules say one must be installed in the caravan as some sites do not follow the rules.
But the real problem is testing. Every DIY man seems to think he can do all the work himself without bothering to test. As an electrician I have fitted many new RCD's which have failed. And also found many installations where the earth loop impedance is not good enough for the protection in place. I have found faulty insulation and all manor of other faults including the odd mouse which has got into equipment.
I have 5 bits of test gear.
1) Clamp on meter measures volts, ohms, and AC amps.
2) Neon tester 50v, 100v, 250v and 500v lamps used to prove dead. Also proving unit to test it with.
3) Low and High ohm ohmmeter uses 250ma and 500v used to test earth continuity and insulation resistance.
4) Prospective short circuit current and loop impedance meter. Used to ensure brakers will not be overloaded and enough current will flow to ensure they will work with a short circuit.
5) RCD tester measures time and current to ensure RCD is not too sensitive but is also sensitive enough and will trip within the specified time.
I look at the DIY guy armed with neon screwdriver and realise why accidents happen. At best we may get them to use a plug in tester at £50 they do not break the bank and do some of the tests we do. Although lighting a little lamp to say earth loop impedance is between two limits is not really good enough but at least a start.
So many of the rules have arrived because of what the DIY guy has done. Like lowering the speed limit from 50 to 30 because of an accident where the guy was doing 80. Clearly all it needed was to catch the guys doing 80.
It happens all the time. People drill holes in the wall without checking for cables. Is there then a rule made to check of cables first? No we have a rule to install a RCD so the silly guy is not injured when he drills through them. If a thermostat sticks in an immersion heater boiling water can soften the plastic in the header tank so it fails. Are we told to use header tanks able to stand boiling water? No we are told to include a non resettable fuse to stop immersion heater working should the water overheat. The fact this will also fail if the back boiler boils the water seems to have escaped the notice of people making rules.
It also back fires with extension leads galore being used because of the Part P regulations. How the LABC can charge more to check work than we would charge for doing it I just don't know.
The 1950's lights did not have earths as there was no earth on the bulb so it's only use was to ensure the switch did not become live. And plastic switches removed that problem. But as things progressed the florescent lamp started to replace the bulb and for it to work there was a metal strip down the tube which was earthed. Also the ballast was prone to overheat so around 1964 the rules were changed and we had to earth lamps. Oddly the metal strip down the tube has now gone but there is still a risk that the ballast will burn out causing a track to earth by including a fuse in the fitting it will fail safe as long as there is an earth. But other items have arrived on the market including lamps with very thin tubes which are not thick enough to have double insulation so in case the wrong size bulb is used and they overheat they require an earth.
But with a stand alone unit the manufacturer has control. He can fit fuses, use fitting types, and other precautions to stop people doing daft tricks to an extent at least. And my touch controlled lamp would just not work if the casing was earthed.
