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General Vintage Technology Discussions For general discussions about vintage radio and other vintage electronics etc. |
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25th Jul 2005, 8:22 pm | #1 |
Dekatron
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Is my house earthed???
I started a thread about earthing AC radios and this has lead to a discussion on the mains supply.
My house was built in 1932 and although it has been rewired a few times, the last about 10 years ago, the wire leading to the mains in the street and presumably the cables in the street are original. They wouldn't have been made to modern specifications, particularly about earth and neutral. Would the electrician who re-wired have had to make special provisions for this? I am sure he would have done so as he was an electricity board engineer of long standing, I am just interested to know what he would have done. Paul |
25th Jul 2005, 8:38 pm | #2 |
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Re: Is my house earthed???
Assuming the rewire was done properly, modern standards will be met. 10 years ago that would be 15th edition IEE regulations (I think, maybe 16th edition was just about ready then, no important changes from your point of view) There should be a test certificate given to the owner of the property. This states details the various ciruits and results of measurements done on them. These include earth loop resistance.
If you commissioned the work only 10 years ago and there was no test certificate issued then it's possible it was a cowboy job. If previous owners commissioned the work then the certificate could have been lost. Typically, an older house with an underground supply will have the earth connected to the lead sheath of the incoming main cable. Newer houses will use PME, where the house earth is connected to the incoming neutral. Older overhead supplies will use an earth rod. Many overhead supplies have been upgraded to PME though if I had that arrangment I would prefer to have a good earth rod too. Those old paper insulated lead sheathed cables buried in the street will be as good as new provided they have not been overloaded or mechanically damaged. They are just as good as modern cables. Last edited by ppppenguin; 25th Jul 2005 at 8:43 pm. |
25th Jul 2005, 8:56 pm | #3 |
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Re: Is my house earthed???
Oh dear I am sure we didn't get a certificate, but I am pretty confident about the person who did it. Maybe that's what they all say! Actually it was nearer 12 years ago.
Paul |
25th Jul 2005, 9:16 pm | #4 | |
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Re: Is my house earthed???
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It's the same in my profession - I've seen some really poor dental work done by people with strings of letters after their name and that should know better, and vice versa. If you're at all concerned, it may be worth hiring a qualified electrician to test your installation, although be aware of course, that some less honest (yet qualified) electricians will see this as an opportunity to sell you a lot of un-necessary "safety upgrades". Apparently, it's very common to be told that some part of an installation is "illegal" or "dangerous" when what is really meant is that it's actually still as safe as the day it was installed and still perfectly legal, although wouldn't be allowed in a new installation Nick |
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26th Jul 2005, 1:45 pm | #5 |
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Re: Is my house earthed???
Thanks Nick,
Actually I am confident about the installation. The person who did it was an ex electricity board engineer who was actually a bit of an "old woman" about getting everything spot on. He was mortified when we called him back because a socket he had fitted to a very iffy lath and plaster wall came a bit loose. We have used it for 12 years now and it all seems OK! Paul |
26th Jul 2005, 2:28 pm | #6 | |
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Re: Is my house earthed???
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If the guy was ex 'leccy board he should have been able to lay his hands on the kit and do the tests. If he wanted to keep quiet about moonlighting he could have written out a certificate with all the measurements and then "forgotten" to sign it. Like Nick, I have seen the lousy and even dangerous work done by people supposedly qualified to do it right. Including British Gas. Don't get me on to another rant about Part P! |
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31st Jul 2005, 8:40 pm | #7 |
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Re: Is my house earthed???
Paul
Domestic electrical installation work is the simplest of installation systems but when carried out without understanding the effect can be lethal. A survey by an NICEIC electrician will allay your fears. He will issue you with a Periodic Inspection certificate that will give full details of your installation type and advice on upgrades if neccessary. One tip. Be sure to have fitted an RCD device at the mains location. 30ma rated should do the trick. You will not then have to be too concerned about 'earth loop impedance values and such. And a note about TNC (PME) systems. Chances are that your house has been catered for by the Supply Authority. Perhaps even without your knowledge. Such systems have inherent design dangers such as to introduce into your home high voltage/current faults, Chances are you won't be in the bath when such faults occur but be sure to protect against 'supply side induced ' problems. You local friendy electrician will advise. Frank |
1st Aug 2005, 10:00 am | #8 | |
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Re: Is my house earthed???
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1st Aug 2005, 10:45 am | #9 |
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Re: Is my house earthed???
Fourteen years ago, half of my parents house was demolished and rebuilt because of severe subsidence. The insurance paid for the work which totalled £66,000. The house was built in 1929 and the earth was taken from the cast iron gas pipe entering the house, there being no PME etc. As part of the rebuild the original gas pipe was removed and replaced with a plastic one. The electricians ignored the fact that the original pipe had been replaced and proceeded to connect the earth wiring to the copper gas pipe inside the house. Some years later, a fault occurred and it was discovered that there was no earth and had not been ever since the rebuild .
I fitted a 4 foot earth rod and got an earth resistance of 40 ohms which is about what one would expect. Ideally, to comply with the regs, an RCCB should have been fitted but this did not get done. A few years ago, the house wiring came under attack by rats and this kept blowing the lighting fuse. Electricians came round and located the faulty wiring in a ceiling and called Southern Electric to sort out the earthing. They fitted a standard clamp affair that connects the incoming lead covered cable with the domestic earth system, the earth is connected to the clamp. Alls well that ends well, Neil |
1st Aug 2005, 11:16 am | #10 |
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Re: Is my house earthed???
Neil's parents' experience shows how much safety margin is often present in domestic electrics. While the regulations may have an element of belt, braces and safety pins, the result is a very low level of injury or damage due to the permanant wiring. Most so-called electrical accidents are due to abuse of appliances. A fire due to clothes hung on an electric heater is hardly an electrical fire though it's often classified as such.
This doesn't excuse poor wiring but does explain why the death rate from faulty wiring is imperceptible. |
1st Aug 2005, 7:29 pm | #11 |
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Re: Is my house earthed???
I went to an Office Block a couple of years ago to investigate why folks were experiencing electrical shocks from metalwork ie radiators, toilet flushes. computor cases etc.
I disconnected the Main earth lead at the PME Suppy Henley Connector and drew an arc of some 150 amps in value and 2 feet long. Seems that the Neutral/ Earth connection out in the street had come adrift and all neutral currents were using parrallel paths to earth. On another occasion I attended a workshop that had live metalwork everywhere. On this occasion a 'blow' in the companies own supply cable had open circuited the upstream neutral and earth and shorted the live to downstream earth. Makes you shiver does'nt it Frank |
1st Aug 2005, 7:33 pm | #12 |
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Re: Is my house earthed???
Thanks all,
We have an RCD fitted and it is a pain when it goes off, particularly as I have a couple of non-self-starting synchronous clocks about but at least I feel fairly safe with it. I am sure the electrician who did the installation was competent and did a good job, I was just wondering about the existing wiring to the street. We have our houses re-wired but nobody seems to bother about the bit that goes to the street which seems strange when you think of the difference in the appliances in use now. There was an RCD fitted when we moved in, but it had been shorted out to stop it tripping due to the perilous state of the existing installation! Paul |
1st Aug 2005, 7:44 pm | #13 | |
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Re: Is my house earthed???
Quote:
Having said that though, a lot of non-technical people I've spoken to seem to think that, merely because their CU has a "trip", that they are immune from electric shock throughout the whole house. A friend of mine was most put-out when he received nasty one while changing a light bulb in the dark (non RCD-protected circuit, of course). I re-wired this house in 1993 with a 30mA RCD protecting everything, and it's never nuisance-tripped once (and yes, it does work, as I test it regularly). It did trip once when I drilled through a lighting cable whilst putting up a picture rail, shorting N-E. If I'd had a split-load system, I would probably never have known what I'd done, and the consequences might well have come back to haunt me at a later date Nick |
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1st Aug 2005, 7:47 pm | #14 | ||
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Re: Is my house earthed???
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2nd Aug 2005, 9:46 am | #15 |
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Re: Is my house earthed???
My understanding of the regulations is that there are two distinct uses for RCDs. They are not always there to give direct shock protection. The point is that if the L-E loop impedance is so high that a fuse/MCB might not blow if live is shorted to earth (e.g. on some TT systems) then a 100mA RCD is used to cover this.
The split load system with 30mA RCD is a crude interpretation of the newish requirement for protecting people from shock on circuits that may be used for appliances used outdoors. The thinking goes that any ground-floor socket might be used that way. I prefer to provide a special circuit for outdoor use so I don't get troubled with nuisance tripping of the whole ground-floor. But it is interesting to see how the 'improved' regulations may now lead to people being electrocuted when changing upstairs bulbs because they think the RCD is there for them on all circuits! A typical unintended result I would say. The ideal but expensive solution would be the combined RCD/MCB on each circuit - but I have yet to see an instance of these (just the reason listed on the price sheet why they are not popular). |
2nd Aug 2005, 9:38 pm | #16 |
Hexode
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Re: Is my house earthed???
The combined MCB and RCD is known as the RCBO (Residual Current Breaker with Overcurrent protection, or something like that). These are not as expensive as they once were (e.g. £25-£30 from Screwfix), and the more recent versions only occupy one module width in the consumer unit, so they can replace the MCB directly.
I opted for RCBOs on a few selected circuits in a new MK consumer unit recently, and they do the job nicely. They look just like the MCBs alongside, but with a test button for the RCD function. Very neat, and a huge improvement over the whole-house RCD which they replaced. The only slight drawback is that if they trip, it is not possible to tell whether the cause was overcurrent or earth fault. RCBOs from other manufacturers may be different in this respect. |