Electrician detecting why RCD trips

[tony sayer]

Still doesn't explain why the transformer with the poor regulation will make the problem worse?.

Where does this 5Khz come from then?. Pole mounted transformers good for that?.....

[Andy Wade]

Yes, I'm aware of that.

Preferred where? - I've *never* seen one in a house installation.

There's still a risk of the operating coil being shorted by over-exuberant bonding of green/yellow wires, or misuse of the subsidiary sensing earth electrode.

Why?

Exactly, and, as a result, ELCBs are no longer permitted.

No-one has ballsed-up the installation. Only external factors have changed.

Except that they were the type used.

Some advantages of RCDs:

- they provide supplementary protection against direct contact, since they don't care what path the residual current takes to earth (this is of course their main use now, in connection with final circuits used to supply portable equipment outdoors); ELCBs only provide indirect contact protection, so are only of use in situations where Zs is too high to ensure operation of an overcurrent device;

- they can be applied easily to individual circuits or groups of circuits, without the need to split the earthing of an installation;

- they are not at risk of being inadvertently bypassed in the way that ELCB coils can be;

- nuisance tripping resulting from external factors is eliminated or very greatly reduced.

[meow2222]

trust me, it does :)

When you switch something on or off, i changes much faster than 50Hz. It typically gets into the rf band. If youre familiar with fourier transforms you'll see where the 5kHz comes in. According to X_c = 1/ (2 pi f c) as f goes up, i goes up proportionally, and this is leakage i.

Where does transformer regulation come into it? Poorer regulation means larger delta V. Delta V across C is what causes your leakage i.

NT

[meow2222]

where all that stands on the statement... explanation continuum I'm not so sure.

NT

[The Natural Philosopher]

I think I do...

I am at a loss Tony.

If you drag 10A out of a transformer with one ohm output impedance, you get a 10v transient drop.

If you drag 10A out of a transformer with one tenth of an ohm impedance, you get 1v transient drop.

All other things being equal, like rise time etc, you get ten times the transient earth current on the former for a given live to erah capacitance.

Add in the fact that poor transformers also have poor winding coupling and therefore ALSO an effective series inductance in the secondary, and the effect is magnified even more.

OK, the transient load needs to be on YOUR side of that transformer..but in my old house definitely very often switching something on would flip the RCD. Things got MUCH better when they under grounded the supply and instead of a gallon can sized transformer, I had something the size of 4 wheelie bins stuck in the corner of the garden :-)

Until a leaking roof made it all unreliable again. Still, roof gone, old electrics gone.. old house gone eh?

[tony sayer]

So pole mounted trannies are capable of passing 5 kHz then?..

[tony sayer]

Must be difficult living there. Odd you have all those leakage problems . Never seen 'em here in UNATHLIC or whatever you term it;-))

[Chris Bacon]

Nor any amount of snips, either, it seems. FFS, old boy.

[meow2222]

The tf doesnt need to pass anything but 50Hz for the 5kHz to be present on the line. If you wish a different explanation, may I suggest sci.electronics.basics. If there's one thing I'm not good at, its explanation.

NT

[The Natural Philosopher]

first of all, yes, as the inter winding capacitance is fairly large, and you should know that bandwidth is a relative term..its all a matter of how many dB down you care to measure..

secondly the issue with the pole mounted transformer is not its sensitivity to voltage fluctuations upstream, but its sensitivity to load fluctuations generated within the domestic environment.

Now its true that those load fluctuations themselves may also be a function of the line regulation upstream..

[The Natural Philosopher]

UTHAMNILIC.

No, you don't get that problem in urban environments with decent corner cabinets and undergrounded cabling.

Remember our overheads are probably pre war, and many of the feed transformers that come off them.

Looking out the window here I can see a line of wooden poles what used to pass pretty much straight through the space I am sitting at right now, 11KV three phase. Some of the houses here only have tow wire runs to them..still 11KV..but I happen to be on a 'B' road rather than a country lane as far as the Grid goes.

The run goes a few miles beyond and feeds parts of the next village. It goes through woodland. Since the disastrous 3 day outages of a year or two ago, they have at least gone out with chainsaws and removed MOST of the offending branches.

You will understand that with a simple tow wires coming in, earths are not supplied by the boiard..you drive your won copper rod in here!

I did get a coaxial cable from the new transformer though, but I still used a rod to be sure..

[Andy Wade]

In the past, yes, but overhead supply networks have mostly been upgraded with multiple earthing so that suppliers can offer PME earthing on new or upgraded installations, provided that the consumer's installation is suitable.

Indeed there's now a legal obligation for the network operators to provide TN earthing - unless they can't :~). This in the ESQC regulations which in 2003 replaced the former Electricity Supply Regulations 1998:

"STATUTORY INSTRUMENTS

2002 No. 2665 ELECTRICITY The Electricity Safety, Quality and Continuity Regulations 2002 Made 24th October 2002 Laid before Parliament 28th October 2002 Coming into force 31st January 2003 24(4) Unless he can reasonably conclude that it is inappropriate for reasons of safety, a distributor shall, when providing a new connection at low voltage, make available his supply neutral conductor or, if appropriate, the protective conductor of his network for connection to the protective conductor of the consumer's installation."

[Pete C]

Would you agree that mean that fatal electrocution on RCD protected circuits/equipment is equally likely as non RCD protected?

cheers, Pete.

[tony sayer]

Well I was curious how the 5 kHz got there in the first place and due to the voltage reduction from what 11Kv to 30 odd volts, this must have been quite a disturbance let alone the losses in the trannie .

Wonder if the makers test 'em and give a frequency response;-)...

[tony sayer]

I presume by tow wire you mean single phase OHL then down to single 230 volt..

Even so that should make no difference to RCB tripping, your own copper rod..

A co-ax cable. Sure you done mean a bit of single armoured cable with the outer sheath carrying the neutral return?.

Odd but we have equipment at remote radio comms sites fed by supply wire twiddled together when Faraday was a lad, and I've not known one instance where the whole site RCD was tripped.

We do test 'em occasionally as its a requirement for lone working and also to see if the reserve supplies and UPS's cut in which they all do!.....

[Pete C]

Simple and cheap to fix though, not to mention worthwhile.

cheers, Pete.

[The Natural Philosopher]

No, but I'd say that the extraneous deaths from a 100mA instead of a

30mA would be almost zilch.

[The Natural Philosopher]

Well it got better immediately the supply was replaced.

As I say, any instantaneous load applied on the far side of a high impedance transformer, causes a big transient. Transients and capacitance trip RCDs

[Peter Parry]

It means electrocution (indoors) is equally unlikely on either circuit as the type of fault an RCD alone is best at protecting against is very rarely fatal on indoor installations. There is one cause of death (which accounts for about 4-5 of the 10 each year) which an RCD isn't very good at preventing and that is falls from ladders induced by an electric shock.

[meow2222]

Many people take more risks when they know theres an RCD in the circuit. But more significantly, electrocution isnt the real issue. Escape in fires is of far more import.

NT