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Topic Title: 4 pole RCCB appears to have blown
Topic Summary: Farm
Created On: 15 January 2011 02:36 PM
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 15 January 2011 02:36 PM
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Zs

Posts: 2910
Joined: 20 July 2006

hello,

One of my farms has called with a problem.

I did this install two years ago. It serves a workshop of mainly single phase kit and three 3p machines, and a grain drying store with big blow fans buried under the piles of grain. The blowers are not in use at the moment.

4 pole hager RCCB 100A 100ma at the very front end of the installation. Tripped with a big bang. He can push the switch back up and it will hold. But no juice coming through the switch at all now.

The three main fuses (100A and I think 1361) are still good. TNCS, situated outside and then direct through the wall into the problem switch. Power is going into the top of the blown switch on each of the phases. Nothing coming out of the bottom on any phase.

He was using a big old mill when it happened. This is a huge lump of steel which has been very cold and is now at 6 degrees with 12 degrees outside temperature. It tripped lightly once last week but was a simple reset. He says it does have condensation on it and it does get bunged up with milling debris from time to time but he sweeps it out.

After the RCCB there is a 3pn fused box for the workshop and a 3pn fused box for the grain store. Both hager and they have 32A fuses for each phase, via a huge henley block to split the tails I think. Can't quite remember but that's what I would have done. These fuses are all ok. after that each has a hager invicta? 3pn board with the usual breakers.Mostly Bs some C.

What might have happened inside the 4 pole RCCB to cause it to stop working? Do they melt? I'm concerned about replacing it and it going again. Too expensive.

Do you agree that this is an earth leakage/RCD issue, in view of the fact that all fuses are fine and no breakers tripped?

You see, I'm inclined to replace with a 4 pole main switch for an hour or so in order to run some tests on what is coming and going from the suspect mill. But what I don't want to do is to divert a problem big enough to destroy an RCD unit into other fuses.

Biggleswade of course, not round the corner. I've not been there yet.

Luckily we are dealing with a very intelligent and sensible man.

Zs
 15 January 2011 02:49 PM
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alancapon

Posts: 5770
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It depends on the fault current the rccb saw. It is possible that a very high fault current will open the device, but damage it internally so it cannot be closed. It has done its job, it tripped on a fault, and contained the arc from breaking the fault within the enclosure.

You would need to test the outgoing circuits to decide where the fault on the installation is, before you try another rccb. It is likely that an rccb will operate before any fuses on an earth fault.

It is worth checking the PSSC/PEFC (kA) of the rccb against the current available from the supply.

If you just fit an isolator instead, you may start losing fuses. Depending on the rating of the isolator, it may be possible to weld its contacts shut.

Regards,

Alan.

Edit: With rccbs and mcbs, the operating handle is not solidly connected to the mechanism, to prevent the device being "held on" with a faulty circuit.

Edited: 15 January 2011 at 03:07 PM by alancapon
 15 January 2011 04:12 PM
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Ricicle

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Not related to the problem, but I take it the TNC-S earth is not used ?

-------------------------
Empty barrels make the most noise.
 15 January 2011 04:22 PM
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deleted_2_tony30

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Is this a hazardous area, with the dust etc?

Tony
 15 January 2011 04:34 PM
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potential

Posts: 1268
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Originally posted by: Zs He can push the switch back up and it will hold. But no juice coming through the switch at all now.

Zs


Is this RCD the type where the switch has to be pulled right down into the off position before it will latch and switch on?
 15 January 2011 05:20 PM
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slittle

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Zs,

I doubt it's the mill unless something really nasty has happened to the starter (I'm assuming star delta)

Is the site TT'd or have you used the TNCS earth and just put an RCD incomer in there ?? I've seen RCD's go floppy and bang if they loose a neutral ?, How is your farmer confirming he's got volts at the top (is he a voltstick man)

The only other times I've seen them go bang is either a dodgy resistor on the test button, but only when the button's pressed or if there has been a lightning strike on the network nearby.

The RCD is clearly kn*****ed but possibly more so on the control side of it rather than the contacts. You'll only know that if you pull it apart.

BOD & JP know how to contact me if you want to discuss it "off forum"


Stu
 15 January 2011 07:47 PM
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Zs

Posts: 2910
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Thank you so far. Many phone calls and your quick reply was very helpful Alan.

Yes, it is TT'd, by me from the TNCS. To be honest I'm not sure where I broke it but it will be very near the front end and at a switch or connection as I usually break at an SWA gland. EDF changed it to a TNCS when they moved the supply. I was surprised. They gave me a hard time I remember, so I probably made tea and acted dumb.

Big time TT with 4 stakes and 25mm. but both units are linked in terms of earth. I actually consulted with one of you when that was in the design stages. This is all new cable, and of good CSA so the test results are as low as you'd expect. Virtually nothing. They used to have a few rare sheep. Otherwise no livestock. Quite a bit of bonded steel. Does that make a difference to this problem? Where's the earth connection point into the RCD? Or did I miss it?!

I've a feeling they will sort it from their end with collected geeks and clamp meters. Then I will be called to go and replace the switch. Fortunately we are in the realms of people like you and I doubt very much if sticky tape and string will be involved. This man made me an olive to fit an SWA gland when the one that came out of the packet failed. It was perfect.

He has told me that the mill has been taken out of service and is completely disconnected. He has found a pool of water in the back of it.

Stu, He has some kind of AVO on the side. And not a voltstick man but I didn't ask. Whatever you use for tractors and go karts I suspect.

Tony30. The grain store is dusty but that's all. all the switchgear is well away from that.

Potential, nice....That's what I thought first of all but no, this is a hager and a proper burn-out. Shame eh?

I will not go there unless I get booked. But will let you know.


Zs
 15 January 2011 08:10 PM
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slittle

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Looks like it could have been the mill then and the RCD took the full force of nature.

If the mill motor is/was full of water, the short circuit currents could have been interesting when the RCD saw the earth fault and tried it's hardest to disconnect the fault.


Stu
 15 January 2011 08:16 PM
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alancapon

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Originally posted by: slittle
. . . If the mill motor is/was full of water, the short circuit currents could have been interesting when the RCD saw the earth fault and tried it's hardest to disconnect the fault. . .

The RCD did actually disconnect the fault. Unfortunately, it was then not fit for continued service.

Regards,

Alan.
 15 January 2011 08:21 PM
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deleted_2_tony30

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http://www.hse.gov.uk/pubns/ais3.pdf

Dusts can be explosive, so watch out!
Tony
 15 January 2011 08:29 PM
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Jaymack

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Is there any obstruction in the mill? Can it be turned by hand, by an existing belt drive; or Stillson wrench on the motor shaft? (in the correct direction).

If mechanically OK; and the IR test is OK on the cable, starter and motor, replace the RCCB after ensuring the kA fault level is suitable.
Why an RCCB and not an MCCB?

Regards
 15 January 2011 08:31 PM
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alancapon

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Off-topic a bit I am afraid, but following on from tony30's post, here is some useless information (unless you do a lot of quiz competitions)

My favourite chemical reaction:

C6H12O6(s) + 6O2(g) --> 6CO2(g) + 6H2O(g)

For those that haven't seen it before, it is an explosion in a custard factory.

Any chemists amongst you will also recognise it as the oxidation of glucose.

Regards,

Alan.

Edit: It does show how seemingly innocent things (like custard) can be dangerous in the right circumstances!

Edited: 15 January 2011 at 09:09 PM by alancapon
 15 January 2011 08:52 PM
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Jaymack

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Originally posted by: tony30
Dusts can be explosive, so watch out!

How many people appreciate hazardous areas in general and the need for special training, ATEX certification and relevant insurance?

Regards

http://www.hse.gov.uk/fireandexplosion/atex.htm
 15 January 2011 09:12 PM
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slittle

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Alan,

Sorry, I did appreciate the RCD done it's job, just fingers were faster than brain. It would be interesting to get the device apart and inspect the contact faces after such an incident.

As to the custard powder, I can still remember the demo my old chemistry teacher gave us on the subject. It took two tiles out of the lab ceiling

Stu
 15 January 2011 09:24 PM
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alancapon

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Originally posted by: slittle
. . . As to the custard powder, I can still remember the demo my old chemistry teacher gave us on the subject. It took two tiles out of the lab ceiling

Yes. It is a surprisingly impressive reaction.

Regards,

Alan.
 15 January 2011 09:57 PM
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slittle

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Originally posted by: Ricicle

Not related to the problem, but I take it the TNC-S earth is not used ?


It wouldn't be a big issue to use the TNC-S earth for a lot of the installation (as described) subject to the usual rules about sizes of earthing conductors, bonding etc. most farms these days have some much "other industry" you can almost consider them an industrial estate with animals in one unit.

It's only really the livestock area(s) that I would have a problem with and want to TT away from the suppliers earth if it was fed TNCS.



Stu
 16 January 2011 06:56 AM
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normcall

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Isn't the Hager unit one of those that you have to push down then up as it has a fault/overload indicator?
Caught of one of my customers out last week.

-------------------------
Norman
 16 January 2011 09:23 AM
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broadgage

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Could the RCD have got wet internaly and failed from an internal short circuit ? nothing to do with the load.
I have seen this, dismantling of the failed unit might be interesting.
 16 January 2011 10:19 AM
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Jaymack

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Originally posted by: normcall
Isn't the Hager unit one of those that you have to push down then up as it has a fault/overload indicator?
Caught of one of my customers out last week.

I had one of these in a sports centre a few years ago in High Wycombe, a previous person had been experimenting and left a note, to switch off the main board isolator in order to reset an RCBO; but I don't reconcile this with the statement in the OP - "He can push the switch back up and it will hold. But no juice (sic) coming through the switch at all now".

Regards

Edited for - I checked on my photo album, this was a 32A RCBO on a Square D Load Centre for a final ring circuit, I was called out to find out the reason why there was no power on the sockets in the dressing rooms. This make of RCBO and model goes to an intermediate position on tripping.

Edited: 16 January 2011 at 11:04 AM by Jaymack
 16 January 2011 10:54 AM
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perspicacious

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"Could the RCD have got wet internaly"

I've had a MCCB with water in it from believe it or not, capilliary from a line tap and making its way down, along, round and up 20 m of 70 mm2 4c SWA, not along the sheath, armouring or filler, but actually along the copper in one core and out from the lug. I drained it by removing a sliver of insulation.......

Regards

BOD
IET » Wiring and the regulations » 4 pole RCCB appears to have blown

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