Faulty MK 80amp RCBO ?

[markymark2406]

We have just completed a installation of a summer house fed with a 6mm 3 core swa backed up at the board in the property by a 20amp 30ma RCBO, the property is approx 25 meters away from the summer house and on PME earth arrangement.
In the summer house we have a MK board the main switch is 100amp with 80amp 30ma RCBO which feed the local MCBs 2 6amp lighting circuits 1 for outside lights and 1 for internal lights / a 16amp radial circuit for small wall heater another 16amp radial circuit for outside socket / we also have a 32amp ring circuit for a couple of other sockets which run a projector and what ever else, the problem is that when we turn the projector on it takes the RCBO out we have tried the projector in the house and it's fine (the house is covered by an RCBO) so I'm thinking of the RCBO is over sensitive in the summer house but just need to confirm how this can be discovered I guess but testing the speed of the trip ie half/half one/one five/five the trip should not trip at half half if it does then it's over sensitive ?[/i]

 

[Monkeh]

You should have identified an over-sensitive RCBO during testing. Did you actually do any testing?

 

[markymark2406]

Ok a week ago

 

[bernardgreen]

Was this installation designed ?

6mm 3 core swa backed up at the board in the property by a 20amp 30ma RCBO
~~snip~~
we also have a 32amp ring circuit for a couple of other sockets which run a projector and what ever else

Is it a 32 amp ring because it has a 32 amp MCB or RCBO in the summer house CU ?

I am thinking an error in the wiring. If you plug something else such in to the socket where the projector was plugged in does that trip the RCBO ? If it does there is a wiring error. Probably a neutral in the wrong bus bar.

 

[Monkeh]

Was this installation designed ?

Or tested. Clearly neither.

 

[AndyPRK]

Good point bernardgreen. It could well be that.

THere appears to be an main switch & 80A RCD in the sumerhouse, plus lots of MCB's.

A picture would help explain

 

[ericmark]

Neutral - Earth fault most likely.

Close to the DNO head with PME earth will be nearly the same polarity as neutral but as distance increases then the volt drop on cables will mean a larger potential difference between earth and neutral so it will take out the RCD.

 

[ban-all-sheds]

Was this installation designed ?

A CU with 5 final circuits, inc. a 32A one, supplied by a 20A device?

A 6mm² cable for a 25m run?

30mA RCDs in series?


If it was designed it was to a very strange brief.

 

[ericmark]

We have just completed a installation of a summer house fed with a 6mm 3 core swa backed up at the board in the property by a 20amp 30ma RCBO, the property is approx 25 meters away from the summer house and on PME earth arrangement.

That seems OK

In the summer house we have a MK board the main switch is 100amp with 80amp 30ma RCBO which feed the local MCBs 2 6amp lighting circuits 1 for outside lights and 1 for internal lights / a 16amp radial circuit for small wall heater another 16amp radial circuit for outside socket / we also have a 32amp ring circuit for a couple of other sockets which run a projector and what ever else.

That seems crazy why have and 80A and a 32A trip of any type when the supply is only 20A. Also when using MCB's why feed then with a RCBO it would only require a RCD.

The problem is that when we turn the projector on it takes the RCBO out we have tried the projector in the house and it's fine (the house is covered by an RCBO) so I'm thinking of the RCBO is over sensitive in the summer house but just need to confirm how this can be discovered I guess but testing the speed of the trip ie half/half one/one five/five the trip should not trip at half half if it does then it's over sensitive ?[/i]

It would seem you have RCBO and RCD mixed up. Still think an earth neutral fault but with the exception of caravans and boats each RCD in series should be 1/3 of the value to the proceeding one. So in the house a 300mA and in shed consumer unit a 100mA one and then each RCBO would be 30mA. That would be unusual normally we only do that with commercial with domestic at main consumer unit 30mA and maybe for specials like pond pumps 10mA.

There is no real point in a 30mA trip feeding a 30mA trip only exception is caravan and boat in marina and the latter has some really odd systems.

It would be impossible to test two RCD's of the same size is series. Although it should not work this way I know that RCD sockets will often take out the supply RCD when the test button is pressed so only way is with a proper tester. And even with a proper tester there must be discrimination between RCD's. There are exceptions like the X-Pole but most RCD's should trip between 1/2 and full declared leakage. So a 30mA can trip anywhere between 15 and 30mA so with three trips in series it's anyone guess which will trip first.

MCB = A magnetic and thermal device which will open in time once the rating is exceeded and very quickly once either 5x (B) 10x (C) or 20x (D) the rated value is exceeded first being thermal bit second being magnetic bit.
RCD = A device which measures the difference between the live cables (neutral is a live) and once between 1/2 and full difference is detected will open.
RCBO = Both of the above combined.

The RCD will normally open all live conductors (line and neutral) but although both line and neutral go through a RCBO most (not all) single width devices only open the line neutral is left connected.

Even I some times forget neutral is live the L in a standard 13A plug stands for Line not Live.

If it was my projector I would plug it into my PAT tester and see if there is a fault. I would assume you don't have a PAT tester so the question is what do you have that can be used to test it?

I have tried to test with a multi-meter and I know from experience the 9 volt used to test will often not find faults often it needs the 500 volt of an insulation tester or PAT tester to show up faults.

My guess is that a capacitor in the EMC filter of the switch mode power supply of the projector has gone faulty. However what I can't work out is a method where this can be tested using DIY equipment.

 

[ericmark]

Was this installation designed ?

A CU with 5 final circuits, inc. a 32A one, supplied by a 20A device?

A 6mm² cable for a 25m run?

30mA RCDs in series?


If it was designed it was to a very strange brief.

I agree question has to be was this a DIY install or an so called electrician who missed three years of collage?

 

[flyingsparks]

Neutral - Earth fault most likely.

Close to the DNO head with PME earth will be nearly the same polarity as neutral but as distance increases then the volt drop on cables will mean a larger potential difference between earth and neutral so it will take out the RCD.

Think you mean same potential, eric!

 

[ericmark]

Neutral - Earth fault most likely.

Close to the DNO head with PME earth will be nearly the same polarity as neutral but as distance increases then the volt drop on cables will mean a larger potential difference between earth and neutral so it will take out the RCD.

Think you mean same potential, eric!

Yes sorry spell check selected wrong option. Could of course be one RCD more sensitive than another and with so many RCD's would be hard to test.

 

[AndyPRK]

I think there are only 2 RCD's in series. But who knows.

 

[ban-all-sheds]

I agree question has to be was this a DIY install or an so called electrician who missed three years of collage?

Ok so i have NVQ part3, i have the 17th edition, shortly to be undertaking testing and inspecting course and exam.

Although judging by his body of work here he didn't learn much when getting those qualifications, or subsequently.

Maybe contrary to your suggestion, collage is actually how he's been spending his time...

 

[ericmark]

I have to admit some of the things taught in collage were rather dubious. I remember being given a digram of a florescent fitting with no choke. It would either go bang or fail to work without a choke and this was level 3 i.e. "A" or advanced level.

This was actually in the book for the course not just a lecturer oversight. Published on behalf of a national examination body.