MCB in van and EHU post tripping randomly

[rollingbob]

Hi everyone, my outfit is a Lunar 524 2010. We bought the van in 2013 and since then the MCB protecting everything except the electrical heating system trips, when we are on site, every couple of days. The site EHU post invariably trips at the same time. I am aware of the need to avoid overload so have been looking for an association between what equipment we are using in the van and the MCB tripping out. We have failed to find any association and the trip appears to occur when we are running nothing except lighting and a fridge. We generally use gas for heating. Note that the RCD never trips. I understand that a short circuit between live and neutral can cause an MCB to trip. Is there any way to check for such a short circuit without dismantling the caravan? Do fridges commonly cause such a problem? At its last service the main dealer suggested replacing the offending MCB itself. This was done but there was no improvement.

We have a very recent (began 2 weeks ago) problem with our Thetford fridge producing an error code indicating that the control panel had lost contact with the power board. The error code disappeared eventually and after much flashing of the blue light, the fridge settled down and worked perfectly. I am thinking of paying a caravan engineer to replace control panel.

I'll be very grateful for any suggestions.

 

[ProfJohnL]

Some of teh MCB's used in caravans are of a relatively poor quality, and can start to trip erroneously. For the simplicity of the work I would try replacing the MCB. BE warned they are sometimes a unique model for caravans.

 

[Woodentop]

If the MCB is a DIN rail mount type (Google it) then they are quite easy to change mechanically but may present some wiring issues. Buy one on the understanding that you can return it if it is the wrong size - the only place advertising that they sell double pole MCBs is City Electrical Factors. You will need one of identical rating - which will probably be 10A and almost certainly Type B.

The wiring issue is that many of the power boxes use a form of insulated busbar from the output of the RCD to the input of the MCB and when you replace one the connector holes may be in a different place. If this is the case then the only option is to replace the busbars with flex cable - 2.5sqmm. DO NOT use domestic mains wiring cable as vibration can make it break over time.

 

[rollingbob]

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?

 

[VicMallows]

From your description you have an intermittent low resistance short-circuit occurring somewhere on the circuits protected by the MCB which is tripping. As you have replaced the MCB AND you say the MCB in the bollard is also tripping, this is pretty conclusive that you do have (an intermittent) real fault. It is also very helpful that you have noted that the RCD is NOT tripping, as this confirms the fault must be directly between L & N, and is not due to a cable shorting occasionally to ground .... which would be a far more common scenario. From you description, it appears that the fault has always cleared by the time you reset the MCBs - is this correct?

There is no real way you can trace this fault by electrical testing. (If the RCD were tripping, then it might be a possibility). You will have to find it by a process of elimination and then visual inspection or replacement.

I would start by isolating every appliance you can on that MCB. Unplug the fridge (use gas if you need the fridge).
Unplug ALL appliances including any microwave. The only things you now have connected should now be the mains lights and the battery charger, and either could well be the culprit. It is conceivable that even a light bulb could be the problem, or the wiring in the lamp fitting as this is often very cramped. If the lamp is adjustable, these are notorious due to twisting, and it may be worth dismantling to check the condition of the fitting wiring.

Just as an outside possibility could water (either rain or a fresh-water leak) be dripping occaisonally onto any mains sockets/connectors?

 

[VicMallows]

Just to add that if unplugging the fridge DOES prove that it is the culprit, I think that an intermittent short in the heater element would be the most likely cause, rather that the control panel. This could well explain the intermittent nature of the fault, as the element expands and contracts during the on/off cycles.

 

[ProfJohnL]

Just to add that if unplugging the fridge DOES prove that it is the culprit, I think that an intermittent short in the heater element would be the most likely cause, rather that the control panel. This could well explain the intermittent nature of the fault, as the element expands and contracts during the on/off cycles.

Hello Vic,

i would agree with the diagnosis of the problem being likely related to a particular appliance or a light bulb filament. but I'm not so sure about it being a heating element fault. , In all the time I have been involved with encapsulated electric heating elements as used in fridges, water and space heaters, Except where the element has been physically damaged, I've never come across one that's gone short circuit L to N. The physical design makes it all but impossible, the usual failure mode is open circuit or short circuit to the casing and thus to Earth.

the apparnt random nature may be related to the switching of some element and thermal effects or it could be loose wire in a fitting which is moved when some one walks nearby and the vibrations upsetting it

Halogen bulbs in my house have often tripped the lighting MCB when they have blown.

 

[rollingbob]

Thanks to VicMallows and ProfJohnL for their helpful replies. In answer to VicMallows question: yes the fault has always cleared by the time I reset the MCBs. I have the van on my drive just now connected to my domestic mains supply so will try the suggested sequence of isolating appliances. Will feedback on progress.

 

[VicMallows]

Prof,
I agree that a L-N fault in an encapsulated heating element , without also causing an earth fault, is extremely unlikely. (unless the element casing is not earthed AND is not in electrical contact with the heating stack pipe). A damaged/trapped wire to the element is though still a possibility.

Because rollingbob mentioned that he had seen a control board fault code, I was really just concerned that he did not just jump to the conclusion that the control board was therefore the culprit (expensive!).

 

[ProfJohnL]

Prof,
I agree that a L-N fault in an encapsulated heating element , without also causing an earth fault, is extremely unlikely. (unless the element casing is not earthed AND is not in electrical contact with the heating stack pipe). A damaged/trapped wire to the element is though still a possibility.

Because rollingbob mentioned that he had seen a control board fault code, I was really just concerned that he did not just jump to the conclusion that the control board was therefore the culprit (expensive!).

I quite agree.

 

[bertieboy1]

Rollingbob,
You have a high resistance fault not a low resistance fault.This is clear from your description (low resistance equals high current and would trip MCB almost immediately) whereas your MCB is tripping after a few days which means overcurrent value is low.
The fact that the EHU MCB is also tripping at the same time suggests there is nothing wrong with your on board MCB (it's probably the same current rating) and both are operating as they should.
A high resistance fault is much harder to find than a low resistance fault.First thing to do is isolate the fault by eliminating each load separately one at a time,the problem is the time taken for the MCB to trip which means it could take days to discover what circuit/device is creating the fault.
If the fault was low resistance and the MCBs were tripping immediately the time taken to discover the source of the fault would be much shorter.
Alas there is no way around this.
When a light bulb fails and the filament breaks it sometimes falls down and touches the other pole causing a short circuit.Some light bulbs have a fuse in the base to prevent this condition affecting the whole lighting circuit. (It's not fuse as you would normally think,it's usually a very thin wire.

 

[ProfJohnL]

Rollingbob,
You have a high resistance fault not a low resistance fault.This is clear from your description (low resistance equals high current and would trip MCB almost immediately) whereas your MCB is tripping after a few days which means overcurrent value is low.
The fact that the EHU MCB is also tripping at the same time suggests there is nothing wrong with your on board MCB (it's probably the same current rating) and both are operating as they should.
A high resistance fault is much harder to find than a low resistance fault.First thing to do is isolate the fault by eliminating each load separately one at a time,the problem is the time taken for the MCB to trip which means it could take days to discover what circuit/device is creating the fault.
If the fault was low resistance and the MCBs were tripping immediately the time taken to discover the source of the fault would be much shorter.
Alas there is no way around this.
When a light bulb fails and the filament breaks it sometimes falls down and touches the other pole causing a short circuit.Some light bulbs have a fuse in the base to prevent this condition affecting the whole lighting circuit. (It's not fuse as you would normally think,it's usually a very thin wire.

I'm sorry Bertie, but I cant agree with your logic of calling the fault a "high resistance", Until the actual fault is identified all we can do is call it a "fault" Intermittent faults are always the most difficult to track down.

 

[bertieboy1]

Prof,
A number of points -
1.My reply is not logic.
2.The fault is not random it happens every time,it just takes a long time to trip MCB
3.The description of "high resistance fault"is in reply to another post that described the fault as "low resistance" and is correct.
4.The fault is high resistance ( low overcurrent),this is borne out by the length of time it takes MCB(s) to operate,the higher the overcurrent the shorter the time it takes MCB to operate.

 

[VicMallows]

Bertie, I cannot agree with your reasoning:

1). You seem to be assuming that the load on the circuit is close to the rated tripping point of the MCB, and that the MCB has such a slow response time that it takes days to actually trip. The OP is quite clear that the intentional load on the circuit is very low (

 

[ProfJohnL]

Hello Bertie,

I suspect we are victims of mutual misunderstanding on this subject.

I approached the OP's problem from a professional background in electronics, electrics and engineering. As such I am used to the accepted usage of particular technical phrases, these phrases have rather specific meanings.

In electrical terminology a "high resistance" fault would mean the actual resistance of a system was higher than the expected range resulting in the circuit passing less current than is permitted, so your use of "High resistance" in this context is not the accepted usage and has led to this misunderstanding.

You need a logical approach to faultfinding, Sometimes a hunch may prove to be correct but the hunch has usually come from a previous similar experience so its rarely without basis, but it's never proven until the actual fault has been resolved.

In this instance the OP has described a fault has occurred a number of times, but the circumstances are not identical each time, but there are enough similarities to the events which can help to narrow the possibilities, but principally because the fault does not have precise repetition of time, without any other identified causal reason it is still "random".

The OP has pointed out the occurrence of the fault is over a variable number of days, and that when it occurs not only does the MCB in the caravan trip but also the site trip.

If both MCB's were rated the same simply through manufacturing variability the exact tripping current will not be exactly the same there would be a few mA difference, so on small over currents one trip would normally trip preferentially and leave the other still set. But as Vic has pointed out, the fact that two both MCBS were connected in series and tripped it points to a very severe rise in current beyond the trip thresholds of both units. This is not consistent with small overcurrent fault as you describe.

 

[Woodentop]

One bit that I find puzzling is that the fault can cause both MCBs to trip - which in practice should/could never happen.
Is it just possible that there are two faults, one in the caravan that trips the caravan MCB and a second either in the MCB that has tripped that is subsequently tripping the EHU pillar, or a fault with the cable - whatever that could be?

 

[ProfJohnL]

Hello Woodentop,

The fault is a mystery, which it's why we can't rule out anything, but I do think its pretty certain to be a major short circuit, but where and how its caused is yet to be determined.

Bearing in mind it is an electrical fault, which seems to be related to an undefined high current, it would be my recommendation for safety the caravan is not used until the fault is found and rectified.

 

[Dustydog]

Thanks to VicMallows and ProfJohnL for their helpful replies. In answer to VicMallows question: yes the fault has always cleared by the time I reset the MCBs. I have the van on my drive just now connected to my domestic mains supply so will try the suggested sequence of isolating appliances. Will feedback on progress.

Any update for us please Bob?
Some excellent technical answers here :cheer:
I have a Martindale plug in mains tester which can tell if there is a problem with the mains wiring to the socket but not the appliance itself. Has anyone used a professional tester on your Lunar?

 

[ProfJohnL]

Hello Dusty

Yes I hope the OP does keep us informed, feedback is useful for everyone.

The plug in type of mains tester, are fine for basic checks for polarity and the presence of the earth/cpc at 13A sockets. Its unlikely it would facilitate the detection of the OP's intermittent fault.

 

[Dustydog]

Hello Dusty

Yes I hope the OP does keep us informed, feedback is useful for everyone.

The plug in type of mains tester, are fine for basic checks for polarity and the presence of the earth/cpc at 13A sockets. Its unlikely it would facilitate the detection of the OP's intermittent fault.[/quote

Thanks Prof.
It was just a thought that in this Hi Tech era there was some piece of kit that could be plugged in and say ouch

 

[Damian]

Quote " Thanks Prof.
It was just a thought that in this Hi Tech era there was some piece of kit that could be plugged in and say ouch "

There is kit available to test each part of an electrical system, but not as a whole.
An intermittent fault is THE most frustrating thing to find.
It takes a very logical method of isolation of every item connected to the power, then adding each item and testing, having first confirmed the main power supply cables to each connected item are not at fault.

I guess as time progresses everything will become controlled by a computer, much as cars are now, where plug in a computer with the correct fault finding software and it will tell you what is faulty.

One common cause of problems is loose screw connections on the AC circuit as AC power has the ability to loosen screws.

 

[ProfJohnL]

...One common cause of problems is loose screw connections on the AC circuit as AC power has the ability to loosen screws.

Hello Damian,

:huh: "AC power has the ability to loosen screws" really? more so than 12V? So its not vibration due to motion?

Please clarify

 

[Dustydog]

...One common cause of problems is loose screw connections on the AC circuit as AC power has the ability to loosen screws.

Hello Damian,

:huh: "AC power has the ability to loosen screws" really? more so than 12V? So its not vibration due to motion?

Please clarify

Prof
I believe what Damian says is a fact. Isn't there some phenomenon where electrical load cycling may cause loosening of screws? But I am a mere Dog so what do I know .

 

[ProfJohnL]

Hello Dusty

Thank you for your "dogged" determination to make me look up
"electrical load loosening screws" on Google.

Several articles came up, some even related to the topic in question, but non of the ones I looked at indicated that load current loosened screw, only that vibrations may loosen insufficiently tightened fasteners.

In fact one article in particular;
http://iaeimagazine.org/magazine/2015/01/15/inspecting-electrical-connections-for-proper-torque/
was of interest.

It confirms that poorly made connections are a real fire hazard, and in this instance the author advocates the practice of torque checking the tightness of fasteners.

So there is no concern about the security of fasteners that have been correctly tightened!

 

[Dustydog]

I knew I could count on you Prof :cheer:
I'm getting ready for next week's departure and had visions of spending ages tightening loads of electrical screws hmy: