Under floor heating control programmer burnt out

[Fluffster]

A client who I am working for has a problem with her electric underfloor heating about 20msq under travertine marble in a basement kitchen.

After the cold weather last week she returned to her house to find the programmer had burnt out internally with carbon staining all over the pcb.
The 2.5mm cable from the consumer unit had the insulation sheathing melted around the contacts at the back too.

The unit is on a 20amp mcb and also an rcd. These had not tripped.
The wiring in the control box which is fed by the programmer for the underfloor circuits appears all correct and undamaged.

Any ideas as to what may have caused this to happen please?

The system was installed over a year ago and used on a daily basis during the colder months for about 8hrs a day.

Thanks
Pete

[sparkyspike]

Do you know how many watts the mat runs at? If it's a 200W per sqm mat, was it fitted with a contactor? The contactor would allow more current to be used than the thermostat could otherwise handle. If a contactor hasn't been fitted, then the thermostat could have been overloaded, hence burning itself out. The 20A mcb or rcd would not necessarily trip.

Has a 13A fcu been fitted, or a 20A DP switch?

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[Fluffster]

Thanks Spike much obliged.
I will advise her.
There is only a 20amp mcb no dp switch or fused switch inline either.

Pete

[sparkyspike]

Ok. If there's no fuse other than the 20A mcb, then it is likely to be a 4kW (17A) system, which will require a contactor. The thermostats you get in those programmers can only handle up to 13-15 amps. As the installer omitted the DP switch, then he probaby omitted the contactor too!

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[ban-all-sheds]

Did the installer provide a certificate to say that it had been done in compliance with BS 7671?

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