UNderfloor heating and rcd trip

[donmazzei]

hi am really new here but was wondering if you guys could help with a lil problem i have??

put in underfloor heating in my kitchen and wired it all up checked the resistance between the caqble all looks good and done a continuity test between the metal braid and the cable for the matt make sure it hasnt been cut during the tiling and that looks ok as well

now heres the problem its connected to a fused spur then to a seperate rcd and then to the thermostat

when i turn the spur on the entire power of the house turns off turn the spur off and i can rest the rcd on the main board and power comes back on

heres the wierd thing i take the fuse out of the spur turn the spur on and mains trips again

so took the spur off and connected it all via terminal blocks and rcd on the main board trips the power off?

is there a earth leakage fault???

also the metal braid on the heating matt is not connected to anything and the mains supply from the rcd for the earth is not connected either to the thermostat since there is no earth terminal on the thermostat

do i need to connect the earth from the rcd and the heating matt to the metal mounting box on the thermostat ??

any help would be much appricated

thanks

tony!

 

[Octopus]

For a start if both of the RCD's are 30mA you need to remove one.

If the RCD on the main board trips when there's no fuse in the FCU sounds like the problem is between the CU and the FCU.

What do the installation instructions say about the earth?

Did you do the install and if so is it your house?

 

[KFH]

It is difficult to comment without seeing the installation and the installation instructions. If the RCD is tripping wihtout a fuse in the spur I would suspect a netral- earth fault. I would have thought the mat needs to be connected to earth. There is no discrimination between RCDs unless one is time delayed so usually the quickest will trip first. What are your Insulation Resistance values? Have you got the polarity correct? What is your Zs at the spur?

 

[Dunc]

If the RCD on the main board trips when there's no fuse in the FCU sounds like the problem is between the CU and the FCU.

Could also be a neutral to earth fault on the underfloor heating element as most FCUs are double pole and thus the neutral and earth would still be connected when the switch is closed.

First things first have you got a insulation resistance tester that is capable of tests at 500V and 1000V? If you don't you won't be able to test the heating mat properly for faults. If you do, disconnect the cold tail to the mat from the thermostat, twist the live and neutral together and do a 500V test between L+N and earth. If this shows a measurement of >299M ohms then do the same again at 1000V. The 1000V test will show up smaller faults that the 500V test may miss.

If the mat tests out okay then the problem is elsewhere. The installation should be connected so that a supply comes from the consumer unit to a FCU (this is assuming you have 30mA RCD protection at the CU). The FCU will then supply the thermostat which in turn has the mat and the under tile probe connected to it.

If there is no earth terminal on the thermostat then the earth from the FCU and mat should be connected to the back box. Also check for crushed cables behind the thermostat, they can opten be deep units and will require a 47mm back box to ensure all connections fit properly without being damaged.

 

[Octopus]

Could also be a neutral to earth fault on the underfloor heating element as most FCUs are double pole and thus the neutral and earth would still be connected when the switch is closed.

First things first have you got a insulation resistance tester that is capable of tests at 500V and 1000V? If you don't you won't be able to test the heating mat properly for faults. If you do, disconnect the cold tail to the mat from the thermostat, twist the live and neutral together and do a 500V test between L+N and earth. If this shows a measurement of >299M ohms then do the same again at 1000V. The 1000V test will show up smaller faults that the 500V test may miss.

If the mat tests out okay then the problem is elsewhere. The installation should be connected so that a supply comes from the consumer unit to a FCU (this is assuming you have 30mA RCD protection at the CU). The FCU will then supply the thermostat which in turn has the mat and the under tile probe connected to it.

If there is no earth terminal on the thermostat then the earth from the FCU and mat should be connected to the back box. Also check for crushed cables behind the thermostat, they can opten be deep units and will require a 47mm back box to ensure all connections fit properly without being damaged.

My "response" was a bit of a taster, trying to get a feel for the OP's expertise.

Many people on this site moan about not getting enough work, yet others go out of their way to support the home DIYer. Lets not forget its kitchen electrics and therefore notifiable.

 

[Dunc]

My "response" was a bit of a taster, trying to get a feel for the OP's expertise.

Many people on this site moan about not getting enough work, yet others go out of their way to support the home DIYer. Lets not forget its kitchen electrics and therefore notifiable.

Very true. However, asking if the OP has an insulation resistance tester will likely be a dead give away as to whether or not he is competent enough to carry out the work. My initial thought is no considering he's only done continuity tests and has no idea what to do with the earth. Time will tell but at some point i'd wager that someone will have to tell him to contact an electrician.

 

[Spudnik]

My "response" was a bit of a taster, trying to get a feel for the OP's expertise.

Many people on this site moan about not getting enough work, yet others go out of their way to support the home DIYer. Lets not forget its kitchen electrics and therefore notifiable.

The man is asking for a little advice.

I would say, that if he is having issues, then he should call a local competent electrician to carry out various test to establish what has been done.

However, yes, this work IS notifiable to LABC due to Part P.

 

[Johnny-G]

...Lets not forget its kitchen electrics and therefore notifiable.

And electric floor heating so it's double notifiable, yay \o/

 

[nikmet]

Where abouts in the uk are you? you may get someone local from here who can help?

 

[philipb]

Hi Dom,
You need to do some basic checks,first disconnect the heating mat completely,turn on the supply and the rcd
unit and see if the rcd turns on and use the test button to test the rcd unit.also has the heating mat been
turned on at all?If the rcd turns on ok with the matt disconnected then either the wiring at the rcd
is the problem or the heatiing matt has failed.A common fault with under floor heating is the connection
at the end of the matt is left to open air or is not submerged in adhesive causing it to overheat
and burn out.

 

[donmazzei]

hi guys thanks for all the tips, ive done a insualtion test and that all seems ok, narrowed it down to the rcd even with no load on the rcd and no fused spur and no fuse in the rcd it still tripping the mains board, so going try another rcd i have in the houe to see if that solves the problem

 

[NICK C SPARKS]

Have you checked pol