3 phase heater RCD tripping

[H. Neary]

Not quite DIY but:

Three phase 415V through RCD/ Breaker unit [3 input terminals, three outputs and one neutral out. Neutral in is via a flying lead [Taken to incoming supply]

From the output The cables go to a six pole contactor.

From the contactor the feeds go to a three element heater block.

6-8kW. The common connection is the neutral which is taken back to the RCD/ Breaker unit.

The earth leakage part of the block trips instantly when the contactor is made.

There is no leakage and no connections other than those described to the phases or neutral after the breaker unit.

There isn't a component at fault it seems to be a design issue.

Anyone any ideas?

regards

HN

[Harry Bloomfield]

H. Neary explained on 17/12/2014 :

Despite your suggestion that there is no leakage, I would suggest there must be. Does it still trip with the heater disconnected from the 3ph supply?

[Chris Hogg]

What are the heater elements made of? Are they bare wires (as in a pottery kiln, for example) or are they mineral-insulated metal-clad elements, such as in a domestic cooker or immersion or storage heater. If the former, then damp brickwork in contact with the elements can be sufficiently conducting to trip a breaker via earth leakage. If the latter, then the mineral insulation can fail with the same result.

[Andrew Gabriel]

What have you done to establish there is no leakage?

Have you PAT-tested the heater?

What type of heating elements are in the heater?

If the heater is connected with a plug, does the RCD trip if you unplug it?

What's the trip rating of the RCD?

[H. Neary]

No I have Meggered the lines and disconnected all but one phase and the neutral after the RCD. It doesn't trip until current is drawn.

HN

[H. Neary]

Leakage is in G Ohms [Lots of em!]

HN

[charles]

Is there an imbalance between the load on each phase? You don't mention an earth lead, but you refer to "Earth Leakage". Don't these devices work on imbalance nowadays?

or

Is the RCD faulty?

[H. Neary]

30mA

supply ---- RCD/ Breaker--- 6 pole contactor----heater[3 elements star connected]-----common neutral--------back to RCD-----supply.

No leakage of any sort. Instant tripping when the contactor pulls in.

The heater isn't a portable appliance, there is no plug. The RCD/ Breaker combination is a Shneider

That's the RCD bit. It bolts on to a dedicated breaker.

Insulation test at 1kV

HN

[H. Neary]

No I have three all in the same config, all doing the same thing.

The breaker section is just that, a breaker. Although I did a check of resistance [identical loads incidentally], I dont think the breaker discriminates against imbablance. I assumed that whatever the imbalance as long as the sum of the currents add up, then the RCD will not trip.

The wiring is good & tested. Everything goes out through thr RCD and the only route back is via the neutral through the RCD.

I'm wondering about the use of a six pole contactor on the output though. I may try feeding the RCD/ Breaker from this, in case the switching transients are giving problems. I'll try forcing the contacor on prior to power up to see if that stops the tripping!!

HN

[Dave Liquorice]

I wouldn't expect it to that what RCDs are for...

Unless the RCD is faulty (swap it with one of the others you have) that would seem to be correct.

Who has checked the wiring, the person who did it or someone else? It's quite common to see what you want to see or what you "know" to be there, not what really is there. Plenty of scope with a 6 pole (so presumably 12 terminal) contactor to get a wire in the wrong place.

[Chris Hogg]

Probably not relevant, but very many years ago we had a piece of kit at work (a 100KV electron microscope) that needed its own earth rod independent of the normal supply earth. This necessitated its own ELCB (don't think RCB's were around then), but it tripped every time the microscope was switched on due to transients, so it was impossible to get the microscope up and running. I think eventually the ELCB was simply bypassed for switch-on purposes, and then brought back in when the microscope was running, but I may be wrong about that. They could have simply (and quietly) done away with it permanently!

[H. Neary]

Yours truly checked the wiring, I didn't wire the units, but I had another look today.

Neutral, 180degrees, embarrasment! Should have spotted it instantly. I will say no more.

I,m off to cringe in the corner. Thanks to everyone for taking an interest, and to yourself Dave for hitting the nail quite soundly..

HN

[Dave Liquorice]

I bet pretty much everybody in here will have done something similar at least once. I know I have. A fault like that is going to be something basic and fudemental, at least you know the RCD works. B-)

[Martin Bonner]

... and the rest are lying about it.