MCB in van and EHU post tripping randomly

[VicMallows]

The AC supply in itself doesn't cause screws to loosen. However the vibration at 50Hz caused by electro-magnetic effects in transformers (in particular) certainly can. It is particularly likely when a screw connection block (chocolate bar) is mounted directly on a power transformer in the 50-500W sort of range.

Loose connections in domestic plugs/sockets are common, but are due to mechanical creep. Stranded wire which has been 'tinned' with solder is particularly prone because the solder is soft and compresses over time. Better practice is to use 'boot-lace' crimps ....or simply leave the twisted wire bare. A more fundamental issue is that in very many domestic wiring accessories the grub-screws/fittings have a thread which is much too course, and often very poorly machined. Ideally they should be a much finer thread with an appropriately shaped contact face on the screw.

 

[BryanTheSnail]

I too have a background in electrical engineering, but resolving problems from miles away is a doubtful expertise.
.So, look at possibles. Could this be a truma electric water heater element? I have had one which failed and sure enough it tripped the van and the whole site. Ouch. But the heat in the fault could dry out the fault enough to let you reset, but only once or twice. That is where I would start. Best advice I have seen so far is drop off one appliance at a time and wait and see or start with just one connected and add if it stays on. This can be done on the drive. Also mentioned, there may be more than one fault and when fault finding electrics NEVER ASSUME THE FAULT YOU FOUND IS THE ONLY ONE.
If you pay for this work it will probable be tackled in this way and be time consuming and hence expensive.

 

[ProfJohnL]

Thank you Vic,
Again its not the current it's self that loosening fasteners, which is the statement we had

 

[rollingbob]

Thanks to all the folks who have taken the trouble to reply. I have been away from my desk attending to some urgent family business so I have not been active on the forum. However I have had time to do some investigations:
1. I have left my van, on my drive, connected to a mains cable (the same one I use on-site) for nearly two weeks with the fridge on but no other appliances. There has been no MCB tripping. This should eliminate the mains cable.
2. I inspected the area around the mains box containing the MCBs after first turning off the mains supply. There is a grille allowing access to parts of the wiring loom adjacent to the mains box. Looking through the grille, I could see two absorbent cloths that had been place underneath the two large cables that run to the independent fuses (not MCBs) for the water heater and space heater.
What were the cloths doing there I wonder?
3. Inspecting the same general area from underneath the caravan I noticed that there appears to be a retro fitted addition earth wire that has been pushed roughly through sealing mastic which protect other cables that run underneath the caravan.. This mod has disturbed the mastic so that there is a smallhole in the caravan floor which would, in theory, allow water ingress from, a wet road surface while the caravan is in motion. Could this be why absorbent cloth has been placed around mains cables? Photos - follow the link

 

[Anseo]

Photos - follow the link

I don't see a link to photos ?

 

[rollingbob]

Link for pictures: www.dropbox.com/scl/fo/tt14a1az7ijhvmmfums9t/AAAlDVPFkbGouEmeEeS8bSfga?dl=0&oref=e&r=AAbiCdM_O0kavp1DcRwL1eKZevxurNJbG7dl8k5tYnfwzz5pqZLDUIgoYZf-R00SPAJrhBIa4XBTiqW1bf8zmL7dnJgQPfE4fD_L9J11YfW1IuuisW_5mesFYZQQ5ryZFTiVwWAlPJUQB5Q8cnHUtB9_Jve7QMtTQN2c4oPbpBhsJi9Qh_8zUL-7-WobkoZUIq0&sm=1

If this doesn't work, I can email access to my 3 photos on this topic on dropbox if you send an email address (via a moderator?)

 

[WoodlandsCamper]

If this doesn't work, I can email access to my 3 photos on this topic on dropbox if you send an email address (via a moderator?)

Or you could use the details in this sticky which shows you how to add photos to this Forum.

 

[rollingbob]

Here are the three pictures uploaded via Photobucket:

 

[VicMallows]

rollingbob,

You seem to have reduced the probability of any static faults due to the fridge. This is not absolute because a vibration-induced fault is still a possibility, even if remote. (from your original description there was never any reason to suspect the hook-up cable.).

With respect, you do seem to have now fallen into the trap of randomly looking for 'possible' causes, rather than systematically tracking down the actual fault. I suggest that your next step might be to unplug the fridge (to eliminate that for the moment); turn on the 230v lights and generally 'wiggle' them and 'stomp around'. (For absolute safety, you could wear a rubber glove when fiddling with the lights). Another 2 weeks I know! .... but that's often just the way with intermittent faults.

In relation to the potential possible damp area you have discovered, I consider this unlikely to be the cause of your problem as described .....it would be far more likely to result in an RCD trip rather than a massive current overload tripping the MCB's.

 

[WoodlandsCamper]

Thanks again. My local main Lunar dealer has already replaced the MCB as I mentioned in my post. It has not solved the problem. The MCB trips just as frequently. Are you suggesting replacing it again or that there may be a wiring issue about which they are unaware?

Have you tried replacing the MCB again? Try obtaining one from a different source as there may have been a bad batch. Your 3rd photo looks very much like our 2012 Lunar and about 2 years ago we had an MCB that tripped out after about 5 minutes when the heater was on 2000W setting, OK on other 2 settings. We could run all day on 1000W setting with no problems, but again after about 5 minutes on 2000W would trip again. A replacement MCB cured the problem.

Edit - the PSU in the back of the unit also needed replacing about 6 months later.

 

[VicMallows]

WoodlandsCamper,

I can well believe that you had a problem with your MCB tripping when you were drawing (at least) 2000W. That is pretty close to the threshold of a 10A MCB, especially if the mains voltage happens to be near its upper limit. There will always be manufacturing variability in the precise trip current. I have put my heater on a completely separate additional MCB for precisely this reason.

The OP was quite specific that there was minimal load at the time of the random trips. Also, how would you explain the bollard MCB tripping at the same time?

 

[rollingbob]

You're right Vic. I've been put off the strictly logical testing route due to the time scales involved. Will do as you suggest and report back but it may take 3 weeks before I can report progress. Happy to leave this topic open. I'll review from time and respond to any posts I see.

 

[rollingbob]

First results of systematic testing now in ! After three weeks on the drive with no appliances switched on, no tripping occurred. I checked the mains light bulbs be removing them and shaking them before replacing after which they operated normally

Then with the microwave plugged in, I stressed it by repeatedly opening and shutting the door and turning it on to a full power cook programme for a couple of seconds and going through this cycle many times. After about twenty cycles, when I closed the door and before I started a cook programme the MCB in the caravan tripped and it also tripped the socket MCB on my house fuse box.
At last I've observed what seems like a cause and effect! A bit of research showed that the door interlock switch may be to blame:
see section 6.17 http://www.repairfaq.org/REPAIR/F_micfaq3.html
This might explain the intermittent nature of the fault.
I replaced the microwave with a new one and continue to test the van with the new microwave installed.
Because the fault is intermittent, it is still tricky to be sure it is fixed but so far so good.
I may test the old microwave in the house but as a non-electrician I have no intention of doing anything but repeating what I did in the van. I certainly will not dismantle it in any way.

 

[StevieRuss]

Have you clamp metered the supply at the MCB when everything is turned on? That would be my first port of call.

I think everything else has been said really. Best of luck.