RCD he say 'click' ?

[Lurch]

On Sat, 15 Jan 2005 20:35:24 +0000, Dave Stanton strung together this:

Could be worse, I've installed loads of faulty ones, apparantly.

[Lurch]

On Sat, 15 Jan 2005 20:13:36 +0000, Mike Tomlinson strung together this:

You might want to check your facts before jumping on the high horse there. You're just as wrong as BW.

[BigWallop]

So this manufacturers diagram is wrong then:

I better tell them about that, because the little man in the drawing will be in really great danger. :-)

[Lurch]

On Sat, 15 Jan 2005 21:39:52 GMT, "BigWallop" strung together this:

I said *most*, I know that MEM and MG have earth monitoring on the RCDs but that's about it.

If the earth connection was required then they wouldn't be the only two in the history of RCD making to have got it correct.

[Lurch]

On Sat, 15 Jan 2005 21:43:14 +0000 (UTC), "Dave" strung together this:

Absolutely none that are relevant to the issues in this thread.

[Mike Tomlinson]

In article , Lurch writes

What's the difference between cutting a length of connected (but not energised) T&E causing the RCD to trip, and the example I posted?

[Andy Wade]

I'm glad you said that. I did too earlier, under the TT system heading. Tony, it depends how good your earth spike is and how 'hot' the neutral is, wrt local ground. The neutral heading back toward the supply transformer is 'lifted' by the voltage drop caused in it by other consumers' load currents, as well as your own.

Here I have a detached workshop building, wired as a TT system. The neutral of the TN-S house supply is not usually more than a couple of volts away from earth, but shorting neutral to earth in the workshop almost always trips the 30 mA RCD. The earth electrode resistance (two

8 ft. rods in parallel) is about 7 or 8 ohms.

[Andy Wade]

It's not that, it's the fact you said "if it doesn't [trip], it's faulty".

That conclusion isn't valid. If you put a lamp in a lampholder and it doesn't light up, does it prove that the lamp is faulty?

[T i m]

Ah, sorry Andrew , can't tell you yet as I got side tracked as Taxi cab for my daughter, first to her recorder session and then with her Fender P Bass and Amp to her band practice. Then getting a Dell Inspiron (lappy) with no floppy or CD reloaded (with no apparent way of influencing the boot sequence to accept std USB floppy or CDROM) with 2KPro ;-(

It does all get a bit deep ... but then there have been some pretty serious contradictions being aired?

All I know is that when I cut the T&E on Dads garage RCD CU it tripped the RCD (it only had a couple of m of wire on it and a couple of flouro's on the lighting cct) ... and the (switched off on the unit) washing machine trips my RCD (not the MCB) when the live is brought into play .. ;-(

I hope to get on it first thing tomorrow .. when I've delivered my daughter and her mates swimming ..

All the best ..

T i m

[John Rumm]

The diagram is right, it agrees with what Lurch said!

The RCD does *not* detect current flow between phase and earth, it can't it is not even connected to the earth. What it detects is an imbalance in feed and return. The fact that in the case depicted in the diagram, the reason for the imbalance was a phase to earth fault (through said loweryesq character) is not actually relevant, any number of other possible reasons could result in a condition that will instigate a trip.

[John Rumm]

You seem to be suffering from IMM syndrome here, repeat it as many times as you like it is still wrong!

Neutral and earth are *unlikely* to be at the same potential, so if you connect them together a current will flow. If this is high enough it will trip the RCD, end of story!

Its behind you...

It don't need to be at phase potential, half a volt will be more than enough!

[John Rumm]

It will only trip if there is a potential difference between N and E, and given that, that the impeadances of each are low enough to allow a current of trip magnitude to flow.

On a TN-S or a TT setup there is a good chance that what you have said is true, the RCD will trip. On a PME system with all circuits disconnected, there is a fair chance the RCD won't trip. You can not infer that the RCD is faulty as a result of this however.

[Dave Stanton]

Good job Part P came in then Lurch .

Dave

[John]

"BigWallop" wrote in message news:I4gGd.11163$ [e-mail]...

Its not WRONG but its too simplified to give a full outline of the operation. What it does do is show to the unenlightened how a fatal shock can be prevented by an RCD. I'm not sure what your level of electrical education is but terms like Vector sum should mean something to you if you are going to "explain" RCDs to the public. Basically the RCD responds to the imbalance of currents flowing in both the live and the neutral conductors passing through the "core" (thats the yellow ring on the diagram). Considering a situation during a half cycle where current flows "out" on the red wire and should therefore flow back "in" on the blue wire. If the two current magnitudes are not equal there is an imbalance and a trip current is unduced in the thin wire sense coil. This current when large enough causes the contacts to be opened. If a neutral to earth voltage exists on the unloaded incoming supply even a small one it CAN be sufficient to give rise to a fault curent flow through a N-E fault which goes "out" on the Neutral but does not come "back in" on the live so the breaker trips. If there is no N E voltage then the situation will not arise. Sometime try putting a voltmeter across N-E on your CSU incomer with the main switch open and see what you get. Depending on external conditions you may get nothing or you could get tens of volts. I once found 40 volts on a supply fed on a long overhead supply at the end of a village street many years ago and long before PME, (earths in those days all came via water pipes). Not enough to do personal damage but certainly enough to give a significant fault current.

[tony sayer]

Arrrrgggggghhhhhhhh!.....

Perhaps you didn't see the other posting where I omitted the finer points of earth leakage via multi return paths in order to try and get the "current balance" concept across to BW and others, without complicating it further!.

Even some pro electricians haven't the correct idea of how an RCD works;((

[Andy Wade]

Arrrrgggggghhhhhhhh what?

What we were pointing out is that while the first part of that is right, the second part isn't necessarily correct at all. Touch neutral to your earth stake and the RCD will trip if the combination of the voltage on the neutral (relative to the stake) and the earth resistance of the stake is such as to allow sufficient current to flow.

[tony sayer]

Yes IF theres a load connected to the RCD and, if theres sufficient induced current over the line from the substation to your local earth due to leakage locally......

-------------------------------===============---------------------

Andy..

It was this bit. Perhaps you missed it?.

Yes you could argue this, but I didn't want to cause any more confusion more than what's perhaps been generated already;)

In fact if there was a heavy earth current into the earth locally than that could be sufficient to cause upset, but its rather unlikely.

In fact I have to alter a circuit thats fed by around 900 metres of overhead low voltage sometime this week I'll see what's induced there.

++++++++++++++++++++++++++++++++++++++++++++++++++

Its quite debatable as to whether or not this will happen. It won't in a PME installation where the earth and neutral are bonded together at the incoming board such as we have here in our office. It may/may not happen where there is an overhead supply, and this will depend on stray leakage's in the locality as I'm sure you will realise.

It was intended to clear up some of the **confusion** as to how a RCD works and try to get across the idea of ""current balance and unbalance"" through the RCD!....

I trust you see where I'm coming from on this now?....

[Dave]

The Panto season is not yet over, so...

Oh! Yes, I did ;-)

An RCD has no recollection of earth, hence my learning of tripping it by earthing a neutral. It didn't take me long to understand why it happened though

I have a limited understanding of it, but I have a greater understanding of the PME system that is in this house. I have tripped the RCD by earth to neutral more times than is needed. (mainly by pulling back boxes over the wire, to change a single to a double socket.)

Don't tell me, you are qualified to do the work, but you are in the position of not being qualified to certify it. What a shambles Part P has become. :-((((

Dave

[Lurch]

On Sun, 16 Jan 2005 23:44:27 +0000 (UTC), "Dave" strung together this:

No, he is not qualified to do either.

[Andy Wade]

No, no, no, no, no, there's no need for there to be any load on the output side of the RCD and no need to talk about induction or local leakage. All you need is some load (said load being in a 3-ph system not perfectly balanced) on the feeder causing current in some part of the neutral between you and the transformer. Now you have the neutral earthed at both ends - one end by the supplier's earth and the other end to your stake, with a current sensor (i.e. the RCD's neutral coil) in series at one end. There are two parallel current paths, one through a dirty great copper or aluminium cable and the other through two earth electrodes and terra firma. Most of the current goes through the cable, but a little bit goes through the earth path and if that's high enough the RCD will trip.

? John Rumm wrote that and I was agreeing with it!

... this has gone on quite long enough.

Whether or not what will happen? We were talking about shorting the neutral to a local earth stake. Whether the main supply is PME or TN-S will make not a jot of difference. But if you're now talking about a short between N and the installation earth then it obviously will. Go back a few dozen articles to wot i writ before:

Trumpington, I thought...