What's this little cable for??? (with pics)

numpty with a crowbar · 5 May 2007

Afternoon folks. About to put an RCD into a little portable distro (32A socket to a CU with an RCD, then a 32A MCB, and then to 3 x 13A sockets on a ring).

I was given an RCD (proteus) to put in it, and it appears to have a little cable coming out of it (can't be much more than .5mm) from just behind the incoming neutral input (the lad who gave it to me isn't a spark so I can't even play dumb and ask him)... But what the hell is it for? I've never seen one on an RCD before and I'm confused..... Any suggestions

This is the top view of the RCD

JohnD · 5 May 2007

functional earth. Connect it to the earth bar.

It ought to have a little plastic tab on it saying "earth".

It does not carry any current in use, that's why it's so small.

MEM have them too. I believe it is to help it trip in the event of a pulsating DC fault.

numpty with a crowbar · 5 May 2007

JohnD said:
functional earth. Connect it to the earth bar.

It ought to have a little plastic tab on it saying "earth".

It does not carry any current in use, that's why it's so small.

MEM have them too. I believe it is to help it trip in the event of a pulsating DC fault.

Ah-ha!!! I thought it might be an earth but didn't want to say it in case I was way off the mark and subsequently scoffed at

Not a trace of a tab to indicate earth or anything... Typical... Much appreciated though - off to wire it in now

ColJack · 5 May 2007

i spy with my little eye...... yellow and green marker in the wire?? top picture? no?

anyway.. it's so that the RCB has a reference to earth for comparing the neutral to..

a lot have them that are not fixed to an earthed din rail...

how do the split load rcb's refference to earth for N-E faults?

Spark123 · 5 May 2007

Anyone got a schematic of the trip circuit for one of these?

numpty with a crowbar · 5 May 2007

ColJack said:
i spy with my little eye...... yellow and green marker in the wire?? top picture? no?

Naah - it's a metal crimp on the end of the wire - just the angle the photo was taken from....

I'm daft but not that daft....

EdwardCurrent · 5 May 2007

Hello All,
What would cause a 'pulsating DC fault' please JD.

Thanks
Ed.

JohnD · 5 May 2007

Spark123 said:
Anyone got a schematic of the trip circuit for one of these?

this is on the side of the RCD

http://i113.photobucket.com/albums/n228/JohnD_UK/MEM_RCD.jpg

Spark123 · 5 May 2007

Thanks
Do you by any chance know what voltage the N-E trip occurs?

lazygit · 5 May 2007

The voltage depends on the impedance of the circuit

V=IR

Voltage=30mA X Zs

(I think thats right)

JohnD · 5 May 2007

Ah, I see what you mean, the diagram shows it measuring Voltage between N and E, right? In addition to the current difference between N and P?

Would have to look at the MEM (Eaton) spec. It might be in the catalogue somewhere.

Have a browse on http://www.memonline.com/f111.html and related pages if you have time.

p.s.that page also answers a question that cropped up recentky "how much does a time delay, delay by?"

ColJack · 5 May 2007

voltage on the earth can rise no more than 50V before the device will trip..

Zs I(delta)n <= 50 V

Spark123 · 5 May 2007

I suppose, will be 25v for some special locs. I was wondering if there was a preset value in the RCD, i.e. it trips at a residual current of 30mA nominal, does it have a N-E voltage nominal too?

lazygit · 6 May 2007

N-E fault will trip the RCD just the same as a P-E fault. As stated the voltage on exposed conductive parts must not rise above 50V or 25V in special locations.
The RCD has no set voltage to which it trips, only an inbalance in current between P-N.
Impedance is what determines touch voltages so for a special location you use the formula mentioned to ensure Zs is low enough.

25 divided by 30mA= 833 Ohms

So provided Zs for the circuit is below 833 Ohms the RCD will trip long before the voltage on earthed metalwork reaches 25V

If you loop test the circuit to obtain Zs you can work out what the voltage will be at the point when the RCD trips. There are some RCD testers that now tell you this info, to save you working it out for yourself

ColJack · 6 May 2007

these types do check the voltage on the earth.. that's what the little cable is for..

they are more prone to nuisance tripping from capacitive coupling and the like..

if you have no inbalance in P-N loading, but still have a touch voltage of 55V on the earth for some reason then it's not going to trip a regular RCD..

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What's this little cable for??? (with pics)

EdwardCurrent

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