VO-ELCBs

[John Rumm]

anyone have one kicking about?

Just wondered what the normal resistance between their frame and earth connections is?

[Andy Wade]

Coil impedance of about 500 ohms for the old black Crabtree one (and close to zero in the tripped or off state).

[John Rumm]

Ah well done that man... I had a photos of the one I ripped out years ago, but could not find any that were close up enough to be able to read it. Pretty sure I binned it at the time alas.

[Andrew Gabriel]

You mean the trip coil resistance? Checking a couple on the shelf...

Henley - 75 ohm Chilton - 62 ohm

These are DC resistance - 50Hz impedance will be higher of course. Neither have anything marked on their rating plates about the coil impedance.

[John Rumm]

I would expect quite a few turns of wire to get adequate magnetic effect from a small current - so the impedance could be quite a bit higher than the DC reading...

[Andrew Gabriel]

Not sure the current has to be particularly small - the coil has to operate before 50V develops across it (or might have been 25V for the test procedure - it's in the back of the 14th Edition Wiring Regs which I can't find just at the moment). Also, these breakers are sensitive to physical shock - banging the wall they're mouted on can cause them to trigger. Some have a two stage trip - the sense coil operates a more powerful relay which operates the trip solenoid.

Current will need to be small enough not to unduely increase the voltage on the earth rod. BS 842 may say more on this if you can find a copy.

[John Rumm]

IIRC we copied the salient bits of the 14th edition into the ELCB article:

The Crabtree one claims a minimum trip current of 35mA. The voltage rise on the earth rod would depend on how its wired I guess. (must admit I have never checked the voltage on one under fault conditions - but I presume that since the bulk of the Ze is from the spike to the substation transformer earth connection you will see near enough mains voltage on the earth local to the property).

It strikes me that the ideal wiring is to have a connection to the primary earth rod direct from the Main Earth Terminal, and this is also connected to the Frame connection on the ELCB. The Earth connection on the ELCB should ideally be connected to its own sense electrode that is outside the resistance area of the primary electrode. Then the ELCB will clearly "see" any voltage rise on the MET, and any parallel earth paths won't prevent the ELCB from tripping so long as they don't manage to hold down the voltage rise (which mitigates the problem anyway)

The way I have seen the connection done on all those I have seen in the wild though, they only have one earth electrode connected on the E connection of the ELCB, and the MET connected to the Frame. That puts the sense coil in series with the earth electrode raising Zs by 500 ohms from the start. Presumably in a case where the electrode Z is say 500 ohm, and that is added to the coil, you only see half the voltage rise of the MET across the ELCB. Probably why the ELCB is set to trip on

17.5V (35mA x 500 Ohm) so as to keep under half the total threshold where it may be hiding half the actual rise by acting as a potential divider.

[meow2222]

ELCBs were from an era in which just twisting CPCs together was considered enough, lighting didn't have a CPC, and speaker wire & Class 0 appliances were considered fine for mains.

In practice things connected to Frame would connect to earth at points - not the earth rod, but earth elsewhere.

NT

[Andrew Gabriel]

It _must_ be connected to its own sense electrode that is outside the resistance area of the primary electrode (which was usually things like the water main, gas main, etc).

A problem was that it might overlap with neighbours' earth resistance areas, particularly after they change something, and false tripping could be caused by current leaking back into it from a neighbour's earth fault. Lightning strikes nearby could do the same.

And that's perfectly fine. If the impedance of the parallel paths is low enough that the frame voltage can't get to 50V, then there is no electrocution risk and the VO-ELCB does not need to trip.

[meow2222]

Yup, its a good thing. Thus ELCBs monitored voltage. F to E current really didnt matter, as long as it wasnt so high as to raise the E rod voltage excessively.

NT

[John Rumm]

It may well be optimal, yet I have never met one connected with a sense electrode - all I have seen to date have just had them in series with the main earth (gas pipe on my previous place)

Indeed. Although in the circumstances where the parallel paths are of a significantly lower impedance than the path through the ELCB, you could be dissipating substantial "leakage" through the various earth paths by then!