RCD tripping still after replace water heater

[Darrencaravan]

Hi,

I have just bought a 2008 coachman Amara. The owner showed it all working. The hot water was very hot. When I got it home when putting on the water heater the fuse keep tripping. I assumed I forgot to fill the tank before checking the water heater switch was off in my excitement to check the van out.

It is an old van so I replaced the water heater and it was very scaled up. I also descaled the hot water tank but still the fuse keeps tripping. I took the wire from the heater and put a plug on it and connected it o an rcd protected extension lead and plugged it in directly to the house. It then worked and did not trip any fuses.

I can only assume the heater heater is fine and that the issue is in the van somewhere.

I am fairly comfortable with electrical work as a mature diyer but could do with advice what to test next.

The heater wire goes to a fused switch. I replaced the fuse in it but was thinking that I need to take the feed to that box and temporarily replace that with a wire to plug into the rcd extension.

Is that the right next step or can you suggest anything else to directly diagnose the fault?

I did notice the fuse box has two types of rcd. It looks like one has been replaced.

The left one is f&s for the water heater, lights and charger. This looks old as it is the same make as the main isolator.
The one next to it is schneider for fridge, fire and sockets.

The previous owner mention he had to have the charger fixed some time ago but it was all fine now and the battery charge is being maintained.

Let me know if I need to provide any more information.

 

[Woodentop]

Do you mean the water MCB (miniature circuit breaker - a switched unit that is effectively an electronic and resettable switch) or the RCD (main switch that kills everything) that is tripping? They perform entirely different functions.

 

[ProfJohnL]

Well there are a number of clues there.. The one that has caught my eye is that one of the 230V mcb's has already been replaced, and I wonder why? Its not very common, but its far from unknown for MCB's to fail by becoming over sensitive. The fact that one has been changed already suggests this might have happened before, and its likely that both original mcb's would have been made at the roughly the same time, and might well have similar failure potential.

A professional electrician should have the necessary gear to test the MCB, but it will probably cost less to simply replace it with a new one (Only buy one from a reputable supplier) Only attempt the replacement if you are fully competent to work with mains electrics.

 

[Woodentop]

Ever had to change an MCB Prof? The issue is twofold:-
The MCB's of different manufacturers vary slightly in size and mounting, so unless you have the ability to change it and modify the system in the process it may not be a good idea:
The MCB's in most caravan power boxes have a proprietary part-insulated busbar system to connect the output of the RCD to the inputs of the MCBs, so again unless the original make is used there will be a rewiring job to do.
And a possible third - modern power boxes can be s**s to get at to do the work.

 

[Martin_E]

The majority of MCB’s are interchangeable between manufacturers and fit into the same space without modification, having said that it is better to use like for like.

 

[Woodentop]

I did a lot of homework on this one and found that a Schneider RCD (Schneider was MEM before they were taken over) was physically exactly the same size as the standard Chinese NBS RCD and thus a good fit BUT the busbar assembly would not and could not be modified to fit. I thus replaced it with a number of pieces of mains flex which worked perfectly.
DIN fit breakers and the like are only specified for the mounting and there can be considerable variation between the aperture size and the unit depth - as I initially found out the hard way!!

 

[ProfJohnL]

Ever had to change an MCB Prof? T....

Yes. and for the reasons you suggest it may need assistance from a professional.

 

[Darrencaravan]

Do you mean the water MCB (miniature circuit breaker - a switched unit that is effectively an electronic and resettable switch) or the RCD (main switch that kills everything) that is tripping? They perform entirely different functions.

Woodentop what I mean is that from the water heater it goes to a switch with a 3 amp fuse. From there it goes to the fuse board. When I turn on the switch it trips the fuse. So as a test I will connect the feed on the switch to a cable with plug into an extension plugged into the house to see if the switch is tripping it. Is that a worth while test?

 

[ProfJohnL]

Hello Darren

There seems to be quite a lot of confusing information in your posts and I cannot be certain you fully understand whats involved.

You haven't told us the make and model of the failed water heater, or of the replacement water heater you have obtained. Presumably the new replacement heater will have come with fitting instructions?

I am surprised the fuse for the heater is only rated for 3A. That would mean the heater would have had an element that was rated to be no greater than 690W, and by 2008 most water heaters were using at least 1kW and would require a higher rated fuse. The actual fuse rating would be in the appliance installation instructions.

In your descriptions as you tell is things are tripping and blowing, but its not clear what you are referring to. For clarity fuses do not "trip", they "rupture" or "blow", you are not clearly identifying if MCB's or RCD's on the Consumer unit (Not a fuse board) are "tripping" (not "blowing") The fuse board in a caravan is for 12V items only,

I am not confident you understand the wiring or the functions and ratings of the fuses, MCB's or RCDs and I must suggest you refer the problem to a professional electrician who has the skill to ensure the systems works and is safe for you and your family to use with confidence.

I'm sorry to be so blunt, but electrical faults or poor practice in caravans can so easily compromise safety.

 

[Gafferbill]

......a 3 amp fuse for a water heater can't be right!

 

[Dustydog]

I can’t find the correct wiring diagram for the OP Amara. The earlier Truma Ultrastore boilers were 450 watts, no longer easily available. Later models were 850w.
Mine is the 850 w via a 5 amp fuse.
I wonder if the OP has replaced the oem element with an 850w one, but sticks with the 3 amp fuse 🙀🙀

 

[ProfJohnL]

I can’t find the correct wiring diagram for the OP Amara. The earlier Truma Ultrastore boilers were 450 watts, no longer easily available. Later models were 850w.
Mine is the 850 w via a 5 amp fuse.
I wonder if the OP has replaced the oem element with an 850w one, but sticks with the 3 amp fuse 🙀🙀

It's certainly possible.

 

[Gafferbill]

.... more modern Truma Utrastores can be 1300 watts.

 

[Darrencaravan]

So for an update and some clarity. The water heater is 850w. I replaced the heater with a Truman ultrasonic 850w. Testing the heater by putting a plug on it and a 3 amp fuse then plugging it into an extension to the house it worked fine.

Then I put the heater back into the next section of wire which goes to the switched fused spur with the 3 amp fuse and taking the out to a cable with the plug on plugging it into an extension to the house it tripped the RCB in the house and blew the 3 amp fuse.

From this I deducted that the issue was within the cable between the heater and the switched fuse spur. So I replaced the cable between the heating cable and the switches fuse spur and replaced the 3 amp fuse. It all worked fine.

I examined the cable that seems to be causing the fault and could not see any visual damage to it. But a happy ending to the issue.

I am sorry if my terminology was not correct to make things clear. I will learn to name things as you do.

Thanks for your help.

 

[Dustydog]

Many thanks for the update Darren. It really helps others to understand the cause of your problem.
My 850 w Ultra store uses a 5 amp fuse . 3 amp is too small imo. But if that’s what your handbook says, fine.

 

[JTQ]

This reminds me that I also once had a EHU cable that developed a fault that would consistently trip out a site bollard's RCD.

I believe this came from what must have been socket/plug area "damp" within what was then an aging cable. As like Darren I could not clearly see a reason why, though in my case a clean-up and chopping off and remaking both cable ends cured my issue.

If a lesson is to be learnt from this it would be to try a different cable if that's viable before assuming the issue is necessarily "van" related.

 

[ProfJohnL]

In #9, I questioned the value of the fuse. Dusty is quite correct the Truma 850W element (infact any 850W element) exceeds the current rating of a 3A fuse which is only rated to handle 690W. Fuses are rather imprecise in operation, so I'm not entirely surprised a 3A has survived. But you should replace the fuse with a 5A to prevent nuisance blowing in the future.

 

[audiorob]

I'm pleased that by your process of elimination you have now effected a cure for the electrical failure. Seems likely, even though not obvious to the eye, that there was an intermittent wiring fault somewhere as you said the previous owner had demonstrated the heater working - so do you now consider that maybe there was no fault with the element that you have replaced? At least now you know you have no limescale build-up on either the element or within the tank - all ready for many years of active service again!

 

[Darrencaravan]

In #9, I questioned the value of the fuse. Dusty is quite correct the Truma 850W element (infact any 850W element) exceeds the current rating of a 3A fuse which is only rated to handle 690W. Fuses are rather imprecise in operation, so I'm not entirely surprised a 3A has survived. But you should replace the fuse with a 5A to prevent nuisance blowing in the future.

I agree with you about the fuse value. I must have been lucky with the one I use while testing. The book does indeed say 5 amp. I have replaced the fuse to the correct value to save any future issues.

 

[ProfJohnL]

I agree with you about the fuse value. I must have been lucky with the one I use while testing. The book does indeed say 5 amp. I have replaced the fuse to the correct value to save any future issues.

As I implied above the manufacture of fuses is somewhat imprecise and there is no definitive way 100% proving non destructively the overburden current needed to cause the fuse to melt.

Under some circumstances a fuse might be able to carry twice its rated current and still not blow. Their real function is to protect the wiring in a circuit from carrying destructively high currents usually caused by short circuits.

MCB's in the consumer unit are usually far more precise than fuses. and again provide better protection for the caravan's wiring.

The other device in a consumer unit is an RCD and other similar devices. These are far more sensitive but rather than protecting from excess current, they detect very small differences in the current flow between L & N conductors of the consumer unit and if the currents are more than 30mA different then the assumption is there is a leakage of current which if it was through a human it could be lethal so it cuts the supply to remove the risk.

 

[toad a caravan]

My carver GE is only 630 w and is on a 5amp fuse as recommended by Carver