What's the easiest way to check for a good quality earth connection at mains sockets?
Am I right in thinking that mere earth continuity is not enough to prove that there's a decent connection to the earth provided by the electricity board?
Daniele
"D.M. Procida" wrote in message news:1hbd9o0.121xowzqazejvN% snipped-for-privacy@apple-juice.co.uk...
Ypu will need and earth loop impedance meter, a low resistamce ohm meter and the skill and knowledge to operate and understand the results.
Shove one end of John Prescott in the socket and a Megger up the other end. Doesn't tell you much, but it sure tells him something.
A "plug with 3 neons" will give you the basics of which socket has the missing earth wire, but if you want to know about "good quality" then there's no option but to get a sparkie's on-site guide (tenner well spent) and hire a real earth impedance meter for the day, then follow the instructions. It's not particluarly hard, but you need the right tools.
This is notoriously hard (and potentially very dangerous) to do. Firstly you don't have a reference against which to compare. You would need an independent earth point, and even then all you would be comparing is the quality of this independent point with the quality of the earth point you're wanting to test. Secondly an earth is only a good one if it can support a short circuit of potentially thousands of amps long enough to allow the circuit protection (fuse/MCB) to cut out. So even if you established that there was a low resistance to earth, this in itself doesn't mean that there isn't a weak link in the earth circuit that would blow before the cutout under short circuit conditions.
Short answer - leave it to the professionals, who (should!) have the specialist and well maintained equipment and experience to do this properly and safely.
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As said, checking earth impedance is not so easy. But all you need is a good enough earth rather than a good earth. There's no need for it to sink 1000s of amps, it only need sink enough to pop an RCD if your install is RCD protected, or enough to pop the biggest fuse/mcb if not.
If you've got a low V bench supply, multimeter, local earth rod and a bit of metal rod, put the rod in the ground some distance form the main earth, and connect the PSU output between the 2 earths. If the output is 'earthed' you need to connect the non-earthed output pin to your 2nd E rod. Now your PSU v/i output tells you the impedance of your 2 earths in series. If both are smimilar rods in similar soil, you now have a rough measurement of twice E impedance. Since your main rod R cant be more than the load impedance seen by the psu, you've got a max impedance number you can count on as well as a best guess number.
The old fashioned way was to connect a 40w mains bulb between live and earth, if it lit up properly the earth was passed. Do not do this, it is dangerous in some circumstances and does not ensure an adequate and safe earth. It does unsafely check there is an earth, and that it passes some current, but thats all.
NT
I did that with a 3 kW electric fire to test the lightning system we have and worked a treat:))
But don't do as I do, and don't do as I say....etc...
================= This method (using a 15 Watt bulb) is described in the current edition (Pub.
2005) of 'Modern wiring practice - Design and installation' by W.E.Steward & T.A.Stubbs so it's probably OK.It's described as a test for polarity rather than earth but the result should be the same.
Cic.
It will of course give any circuit breaker the willies though, won't it?
Daniele
neither mcb nor fuse would take any action. They dont care where the current goes, just that there isnt too much of it, ie >6A, >32A etc..
Its not altogether safe to do with a 15w bulb because if your E impedance is high or o/c, everything in the building thats supposed to be earthed becomes live. This is not a safe scenario. It would be fine if your earthing is ok, and no E connection has come adrift anywhere, but if youre doing this kind of test, its probaly because your earthing isnt behaving as expected.
Daniele why do you want to test earth impedance? Or did you really just mean you wanted to make sure your socket earths were connected, which is a diffrent thing? (much easier)
NT
Mind you that won't be too clever either what with all the "EMC" suppression capacitors coupled twixt line and earth.....
Yes, not ideal. I guess the currents involved should be low enough that almost any form of incidental earthing would keep it close to zero. And the unloaded voltage would be 120 rather than 240.
NT
Really? I thought that that the circuit breakers on consumer units also kept an eye out for current going to earth (are they called RCDs?) and took action accordingly.
Daniele
That *is* an RCD you are thinking of - effectively measures the difference between live and neutral currents and if the difference gets to big (say
30mA is the most common), it trips. It doesn't care about overload current.The beauty is an RCD doesn;t care where the "missing" current has gone, just that it isn't returning via the neutral. So it will detect current disappearing to this circuit's earth, another circuits's earth, another circuit's neutral, down the water pipe, down soem hapless person's feet into the wet grass etc.
Now, more often than not, you get one RCD on the consumer unit which monitors a number (sometimes all) the circuits.
The devices on each circuit are either a fuse or an MCB, teh latter look similar to the RCD in as much as they have a lever to operate them.
MCBs are purely for tripping out over current faults (like fuses).
There is a combined type called an RCBO which does both jobs in one device - you don't see these so often as they cost more.
You're right, if you have an RCD protecting the circuit and you test the earth with anything that draws more than 30mA or so, the RCD will trip. Your test lamp for example.
You can get (expensive) instruments to test the earth that try not to trip the RCD - electricans who don;t have an instrument with the feature have to bypass the RCD to perfrom the test.
If you just want to test that the earth is vaguely connected, buy one of those tester plugs with the 3 lights in them. Quick and easy and safe.
Doesn;t prove the earth is solid - but will show obviously missing earth, and a few other problems too.
For "proper" testing, you really need a specialised test meter which cost in the order of 100's pounds new (depending on whether it only does earth tetsing or insulation and RCD testing too).
Cheers
Tim
Circuit breakers trip on overcurrent and that is all[1]. Most have two mechanisms - a magnetic part for large overcurrents (shorts etc, this usually kicks in at a few times the rating of the breaker[2]). This opens the breaker very quickly under this type of fault. In addition there is a thermal mechanism that opens the breaker more slowly after a period of prolonged overcurrent (e.g. drawing 1.5x the rated current for
20 mins).RCDs are different beasts altogether (but see [1]), and monitor for a difference in the current going out and returning. Hence enough[3] loss to "somewhere else" (e.g. you, earth etc) will cause them to trip.
(there is a another archaic device (use now deprecated) called a Earth Leakage Circuit Breaker - which monitors the current flowing to an earth terminal)
[1] Just to add to the confusion there is a third category of device in reasonably common use called a RCBO. This combines the functions of RCD and Circuit Breaker (MCB) [2] How many times greater than the rating of the breaker the fault current needs to be to cause an instantaneous trip depends on the type of breaker. Type B requiring less than types C or D. [3] Enough will depend again on the rating of the trip sensitivity. Common sizes are 30mA (required for protection of sockets that can power outdoor equipment) and 100mA (often used for whole house protection on systems with a local earth rod).
If your wiring is new enough to have an RCD, that would trip. For most people nothing would trip. Those with partial RCD cover could use the lighting circuit, which will usually be non-RC.
In principle you could still do the lightbulb test on an all-RCD TT system by: a) ensuring equal i flow from L and N to E or b) using an isolating transformer and feeding the output of that to earth and a secondary earth.
But I dont suggest anyone doing it, not a good way to test earth impedance.
NT
"D.M. Procida" wrote in message news:1hbd9o0.121xowzqazejvN% snipped-for-privacy@apple-juice.co.uk...
We use this at work to verify each week the earth loop impedance important work benches.
The brainwashed Tory Little Middle Englanders are still at it. Part P is bill.
You'd get an inf. reading - just a vacuum!
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