Compatability of Underfloor Electric Mat Controllers

[Mel_P]

Hello, my daughter's bathroom has a Screwfix 2 sq m mat I installed a few years ago. When she had her electrics upgraded the controller was replaced with an RTC80.716 controller and it seemed to work. Now however I have 2 issues:

1. it keeps tripping the 6A MCB at random intervals. Perhaps it has an internal fault. I have measured the resistance and it is 179 ohm - should be 175 ohm - so seems OK there. No apparent leakage to earth.

2. She cant be bothered to programme it!

So I propose to replace the controller with a simple manual controller from TLC (BN A16C). This has a 2-wire sensor.

QUESTION - will the TLC controller work with the 2 wire NTC sensor embedded in the tiles with the screwfix mat please? thanks.

 

[Octopus]

How can you say there is no leakage to earth if you've just done an end to end reading?

How can you be sure that it's the underfloor heating tripping the circuit?

 

[Mel_P]

- disconnected heater and heater exhibits no leakage to earth.

- heater is only thing connected to MCB and trip seems to occur randomly with switching of thermostat.

 

[Dave OCD]

What is the current rating of this underfloor mat , a 6a MCB seems a bit on the low side ?

 

[Mel_P]

1.3A measured. 300W which should be about 1.25A

 

[Taylortwocities]

What is the current rating of this underfloor mat , a 6a MCB seems a bit on the low side ?

He's already said the resistance is 179ohm!
I'm not sure why you need to ask about the current rating when simple ohms law will tell you that the mat will pull a lot less than 6amps.

 

[Mel_P]

I have been investigating this today. When I measured its resistance to earth it was initially about 200 kohm but later after a couple of hours operation it seemed to be only 50kohm (these are with a 9V battery DVM). Should I get values of > several Mohm???

 

[Taylortwocities]

Yes you should. A 50k reading to earth should still not be enough to trip a 6A MCB.
But it could trip an RCD, is there one protecting the circuit?

 

[Mel_P]

It's a RCBO B6 30ma

 

[Taylortwocities]

Ah, so not an MCB after all

1. it keeps tripping the 6A MCB at random intervals.

So it could be an earth fault, or an intermittent short circuit then.

You really need a proper insulation resistance tester, as a DVM won't stress the circuit properly. An IR tester would whack 500V up it.

 

[Mel_P]

Isn't an RCBO a combined RCD / MCB ??

 

[Mel_P]

Ah, so not an MCB after all. So it could be an earth fault, or an intermittent short circuit then. You really need a proper insulation resistance tester, as a DVM won't stress the circuit properly. An IR tester would whack 500V up it.

I suspect that using it in the circuit provides me with an insulation resistance tester - the circuit is stressed by the 240V and trips the RCBO. I dont need to splash out £60 min on an Insulation Resistance tester (Tenma) for a one off use!

 

[Octopus]

^ £60k for an IR tester .... And the rest!

 

[Taylortwocities]

Isn't an RCBO a combined RCD / MCB ??

Yes, but your RCBO will trip if there is an overload more than 6A OR if there is an earth fault impedance of 30mA.

At the moment you do not know which fault you are looking for.

 

[Taylortwocities]

I suspect that using it in the circuit provides me with an insulation resistance tester - the circuit is stressed by the 240V and trips the RCBO. I dont need to splash out £60 min on an Insulation Resistance tester (Tenma) for a one off use!

Not really. An IR tester will test the circuit under during stressed load conditions.
You don't know what your fault condition is, only that the RCBO trips.

So, what do you plan to do now?

 

[Mel_P]

Well I tried it on another circuit. One with a 13A "non-ring" socket on on a 16A MCB with an overall RCD on the consumer unit. the MCB did not trip but the 30ma RCD did. Suggests it the earth leakage causing the problem?

 

[Octopus]

tried what on another circuit?

 

[Mel_P]

The whole thing - disconnected mains input to wall thermostat and connected 240v via fused 13A plug / flying lead to thermostat / heating pad. Capped of 240 leads from 6A RCBO during test.

 

[Octopus]

The whole thing - disconnected mains input to wall thermostat and connected 240v via fused 13A plug / flying lead to thermostat / heating pad. Capped of 240 leads from 6A RCBO during test.

That's not the ideal way of safely going about fault finding..