Duel Source Towel Rail Problems

Hi just after a bit of advice, I have been called back about a year after installing an electrically heated towel rail that is also connected to the central heating pipework. The fault caused the RCD to trip when ever the towel rail fused spur was turned on, I checked insulation resistance and found that L to N and to PE were all showing as a full short. The plumber drained the system and changed over the element. I went back afterwards and reconnected the element to the SS and the same thing happened. The new element has a detachable control unit that allows a 2 hour boost and has a thermostat control knob, it all works fine when the control unit is detached from the element, all lights work, thermostats clicks etc, but as soon as the element and control unit are plugged back together the RCD trips. Any ideas where or what to start looking for? The switched spur is part of the 1st floor ring and tests and works normally.

From your description it sounds like the controller has failed

mvkpe1 said:

 I checked insulation resistance and found that L to N and to PE were all showing as a full short.

Click to expand...

If L-N and L-E are showing full short then I suspect your fuse would be blowing NOT the RCD tripping??

Most IR testers cannot show readings low enough to indicate a full short.. 

i.e.  0.00MegOhms is the reading you would get for a resistance of 0.009999 MegOhms,

0.009999 Meg =  9.999KOhms or 9999ohms...

9000+ ohms is not a full short in my book....

You general need a meter with a low ohms range to read full shorts.  

What are you actually calling a "FULL SHORT"

you may have been just reading through the element???

If you had fault symptoms..

changed an item..

Then still have fault symptoms

possibly the item you have replaced was not actually faulty????

Need more info and actual figures.

opcorn

I agree that the test between L&N was probably the resistance of the element I was more concerned with the L-PE and N -PE both reading 0.0M ohms, which would of course trip the RCD.

I would also agree that maybe the original element wasn't faulty, hence why the question about any ideas where or what to start looking for. The circuit in question works perfectly without the element connected.

as stated above, 0.0M ohms may not cause an RCD to trip, and does not indicate a short circuit

mvkpe1 said:

I agree that the test between L&N was probably the resistance of the element I was more concerned with the L-PE and N -PE both reading 0.0M ohms, which would of course trip the RCD.

I would also agree that maybe the original element wasn't faulty, hence why the question about any ideas where or what to start looking for. The circuit in question works perfectly without the element connected.

Click to expand...

If the NEW element also gives those readings, then you have got a second dud element.

but are you measuring that with the bare element, OR with the element connected to the controller.  If the latter the problem is more likely to be in the controller.  Don't forget if you have only isolated via the MCB, then N will still be connected and will read almost 0 ohms to earth You need to disconnect properly to make any meaningful measurements.

Just a silly question, the towel rail is filled with water isn't it?

I had to replace an almost new element as the owner had installed the towel rail and not filled it with water. then wondered why it didn't get hot )perhaps just slightly warm in the corner where the element was) and then not for long (the element burned out)

Yes the radiator has been refilled, and I was testing the element alone and disconnected from the switched spur as well as by the MCB at the DB,

I will have to do some more digging and testing the next time I go back, I just thought that somebody might have come across a similar problem before..

mvkpe1 said:

 I was more concerned with the L-PE and N -PE both reading 0.0M ohms, which would of course trip the RCD.

Click to expand...

No 0.0Meg would not always trip an RCD...

30ma @ 230v = 7666 ohms or lower to trip RCD...

above 7666ohm up to 99999ohms would NOT exceed 30ma..

But would show as 0.0Meg..

1.0Meg = 1000000ohms!

mvkpe1 said:

I will have to do some more digging and testing the next time I go back, I just thought that somebody might have come across a similar problem before..

Click to expand...

Yes have seen a similar problem numerous times before...

Persons not doing all the test readings to allow an accurate evaluation of the fault..

Only using IR tester and assuming 0.0Meg is a dead short!!

:shakehead

As i asked earlier that you haven't replied..

What are you calling "A FULL SHORT"..

to me full short is a fault of negligible impedance ???

:mellow:

mvkpe1 said:

 The circuit in question works perfectly without the element connected.

Click to expand...

Is it a dedicate RCD for that one circuit..??

Or multiple circuits via one RCD...

What are your RCD test readings??

Ramp test value...

etc..

etc..

There is an issue when fault finding on such faults and using say a multifunction tester.

As has already been pointed out the insulation resistance range is not accurate at low values, if you have a lower voltage setting, say a meter with, 50, 100, 250, 500 & 1kV ranges then you have more chance.

The low resistance range will often show open circuit @ > 2 Ohms, so between 2 Ohms and almost 10k Ohms you have no possibility of getting a true reading.

Time to reach for a multi-meter really.