Fault Current

[Willy]

Hi All,

Just a question that's been eating away at me. I'm currently doing a new ring in a kitchen, earthing arrangements are TT, with a Ze of 21 ohms, so far so good. The cu is protected by a 250ma RCD main switch with the new ring to be put on an 32Amp, 30 ma RCBO.

Now the PFC is only 11 Amps, which is obviously not great enough to trip the overload protection in the event of a dead short, so how does it trip the RCD, is there a tiny imbalance at the instance the circuit moves from healthy to short? Or is there another rational explanation?

 

[Andy™]

250mA RCD? are you sure about that?

and your PFC is the highest between live to earth, and live to neutral. TNCS should be identical, otherwise l-n will be higher. 3 phase is phase-phase

if there is a dead short l-n, RCD wont trip. MCB will though. if live-earth, RCD will trip. MCB may trip if fault current is enough (you can sometimes get a Zs of a TT to less than 1.5 ohm)

 

[Zeespark]

Are there non RCBO circuits in the board at all?

AndyGuiness Drink

 

[Willy]

[quote name='Andy

 

[Andy™]

l-n will be much higher than l-e of 11a. and enough to trip MCB. fault current to earth only needs to be 30mA for a 30mA RCD

 

[Willy]

Are there non RCBO circuits in the board at all?AndyGuiness Drink

Yes Andy, there are 2 lighting circuits and a cooker unprotected by RCDs

 

[Willy]

[quote name='Andy

 

[batty]

On TT systems I do a loop test l-n this is what needs to below the amount in osg 1.2ohms roughly for a b32 breaker to ensure it will trip if there is a l-n fault. The loop l-e can be up to 1667ohms as Rcd or Rcbo will take care of any faults on that side. But if you have a loop l-e greater than 220ohms I would look at putting another rod in or looking at alternative ways of earthing.

as Rcd can be unstable at higher readings.

Batty

 

[kme]

Batty - I think you might have meant 1666 ohms, not 16.6K ohms?

Otherwise, I agree with your rationalle

 

[batty]

Batty - I think you might have meant 1666 ohms, not 16.6K ohms?Otherwise, I agree with your rationalle

Not good at exact figures but generally know when something is a miss.

Batty

Sorry have put it right had two many 6's

 

[SPECIAL LOCATION]

Hi All,Just a question that's been eating away at me. I'm currently doing a new ring in a kitchen, earthing arrangements are TT, with a Ze of 21 ohms, so far so good. The cu is protected by a 250ma RCD main switch with the new ring to be put on an 32Amp, 30 ma RCBO.

Now the PFC is only 11 Amps, which is obviously not great enough to trip the overload protection in the event of a dead short, so how does it trip the RCD, is there a tiny imbalance at the instance the circuit moves from healthy to short? Or is there another rational explanation?

As the other guys have identified and you quite rightly say in your Post #7

you threw a bit of a blank at fisrt....

but just as a side note your initial incorrect thought IS the very reason why TT installations had to have RCD's!

Because any Line-Earth fault will not have a sufficiently low impedance to allow an MCB to operate. and thus could leave dangerous voltages at exposed conductive parts if no RCD present! :|

For the benefit of others reading who may not have sussed out what the initial error of judgement was....

With all single phase circuits with Live Neutral & Earth conductors..

there are three possible permutations of short circuit bridge...

1) Live-Neutral

2) Live-Earth

3) Neutral-Earth

As neutral & earth are at near enough the same potential 3) has No effect on an MCB (or fuse)... But will trip and RCD.

1) & 2) Will be near identical on a PME (TN-C-S) earth supplied installation cuz neutral & earth are bonded in multiple places.

1) & 2) Are often very similar on TN-S earth supplied installtions...

But they can be different due to separate earth paths back to supply transformer.

1) & 2) Will vary greatly on a TT installation where....

2) is basically your Ra (earth electrode resistance) value.

and 1) becomes the actual PFC!