Thread: Any sparkys here? Electrical oddity...

Joe,
this is the best place for electrical questions:

ELECTRICAL

Some of the blokes are very helpful. Of course, there's the usual "don't risk it, call a sparky" brigade, but I've usually been able to get the info I need.

Chris

RCDs work on balance - through electrical coils they 'compare' the current flowing in the active and the current flowing in the neutral and if they don't match, trip.
It sounds like (I'm not a sparky) that when you short to earth, a current is able to flow. The RCD is doing what it should. The real question is whether the rest of your electrical is behaving...

Michael

Hi,
It is nothing to worry about. The RCD works on the current in the A and N being = to within the rating (30ma in your case) ie what comes in in one leg must go out the other. If either goes to earth it will trip. it takes very little diference in potential to get a flow of 30ma and that will come from the difference in resistance and or impedance of the earth conductor / path to that of the neutral conductor in a N to E short.
If this does not make sense it doesn't matter the first sentence is the important one "It is nothing to worry about."
Regards

Thanks all. That's exactly what I understood and believed to be the case.
Hugh, the earth conductor in the main cable is 2.5mm sq and the N&A are 6 or 10 (can't remember). Would that account for the difference? Other than that, the length and path to my shed are identical.
Is there any relevance in my SWER connection to this issue?

Michael, yes, everything is working correctly and safely.

Cheers,
Joe

Hi,
No the SWER is on the high tension side of your transformer and should have no effect on the secondary circuit. The different size of the conductor and if there is other load on the way to where the short happens will help account for the slight current flow.
Regards

I think the SWER can produce this effect. Since the ground is the return conductor for the SWER transformer, there could be a potential difference between the transformer and the shed sizeable enough to activate the RCD when the connection is made.

Anyway, why are you doing this?

Tool4me,
I'm not doing anything. I happened to accidentally touched the netral with my screw driver (in a switched off circuit) while tighterning the earth terminal when I swapped a single GPO for a double and heard the RDC trip.
I'm only looking for an explanation.
Joe

Um. You were fooling around with a GPO with power applied to it? Braver man than me, that's for sure.

Originally Posted by petersemple

Um. You were fooling around with a GPO with power applied to it? Braver man than me, that's for sure.

No, he said the circuit was switched off. Typically this only cuts the active supply hence the fact that the two wires mentioned originally were the neutral and earth.

Thanks Karl - no, Peter, I'm not at all brave - nor foolish , you jumped to a conclusion without reading my post.....
Joe

Anyway, I think I might have got an understandable answer from the Resto forum sparkies: the fact that thewe are on a SWER line means that there is a ground path back to the transformer neutral. Since the shed is about the same distance or closer to the transformer than the main earth stake - and the shed is well grounded in this wet weather, it is possible that a small current form any user can go back via the gound, thereby making the currents in active and neutral wires unbalanced - tripping the RCD.
Makes sense to me....
The other suggestion was that the earth cable to the shed is a lot smaller that the active and neutral. That would change the impedance over that distance and could also imbalance the current sufficently.
In any case, nothing trips form any equipment and when it does, there is a good reason (I've had a build up of dust in the bandsaw motor a couple of times tripping it). So all is goos and my curiosity is satisfied.

Cheers,
Joe

PS: don't have any concerns for my electrical safety anyone - I know enough to be scared of messing with electricity, and enough to know what I can and can't do safely.
Sadly, Ihave also experienced what sort of unsafe and dumb things electricians can do if not supervised - like leaving a job at my house with the roof 'live'! I recognnised the fault myself, foudn the cause, got him to ccome back and didn't pay the bill.....
Since then, I made it my business to know the rules and understand domestic wiring properly, so I can supervise. Sparkies hate that - but I can sleep at night.

Originally Posted by jhovel

Anyway, I think I might have got an understandable answer from the Resto forum sparkies: the fact that thewe are on a SWER line means that there is a ground path back to the transformer neutral. Since the shed is about the same distance or closer to the transformer than the main earth stake - and the shed is well grounded in this wet weather, it is possible that a small current form any user can go back via the gound, thereby making the currents in active and neutral wires unbalanced - tripping the RCD.
Makes sense to me....
The other suggestion was that the earth cable to the shed is a lot smaller that the active and neutral. That would change the impedance over that distance and could also imbalance the current sufficently.
In any case, nothing trips form any equipment and when it does, there is a good reason (I've had a build up of dust in the bandsaw motor a couple of times tripping it). So all is goos and my curiosity is satisfied.

Cheers,
Joe

PS: don't have any concerns for my electrical safety anyone - I know enough to be scared of messing with electricity, and enough to know what I can and can't do safely.
Sadly, Ihave also experienced what sort of unsafe and dumb things electricians can do if not supervised - like leaving a job at my house with the roof 'live'! I recognnised the fault myself, foudn the cause, got him to ccome back and didn't pay the bill.....
Since then, I made it my business to know the rules and understand domestic wiring properly, so I can supervise. Sparkies hate that - but I can sleep at night.

I had a qualified electrician install my new switchboard and RCD switch. He is the electician at work. He deals with single and 3 phase circuits every day at work. After 2 hours of messing around he claimed that maybe my RCD switch was faulty. Then he read the instructions that came with it! Problem solved!

Dean

Sometimes familiarity breeds contempt...

Woops, you are correct. I didn't properly read the thread. I can only plead bad cold which is stuffing up the thinking process. Sorry.