are most 80-100a CU rcd's. rcd's or rcbo's?

[ADS]

I'll try to be clearer -

With a L - E fault, NozSpark is correct, the fault is in parallel with the load

and, as Lostit says, the leakage is dependant on the resistance offered by

the fault - it is this that determines how much current travels to earth.

This fault is also 'always there' - regardless of whether the circuit is under

load or not - if it's tripped the RCD, you won't be able to reset it without

rectifying the fault.

But....

A N - E fault is a different story

When the neutral shorts to earth/cpc, they are both at 0 volts (roughly),

there is no current drawn and it won't trip the RCD until a load is present.

The current (drawn by the load) travels down the line conductor, through

the load and returns down the neutral conductor - except that in this case

the N - E fault gives a parallel path back, splitting the returning current

between the neutral and cpc.

The ratio of the split is dependant on the resistances of the two parallel

paths.

This fault is totally reliant on the current drawn by the load/s, as the higher

the current drawn, the more will travel down the earth path.

This is one reason why you can get intermittent RCD tripping -

Customer puts the kettle on - RCD is fine

Customer puts the washing machine on - RCD is fine

Customer puts kettle on while washing machine running - RCD trips

The increase in load has increased the current leaking to earth -

a N - E fault

 

[Lurch]

I must be overly tired or something.

 

[Lee321]

I must be overly tired or something.

i fell off my chair

 

[NozSpark]

Sorry I got the wrong end of the stick before,,, didn't realise you were on about N-E faults....

With N-E faults you don't necessarily need that circuit to be energised for the RCD to trip.. If another circuit is energised some of that current can travel back up the N conductor of the faulty circuit to earth.

 

[ADS]

Quote:

Originally Posted by Lurch

I must be overly tired or something.

i ask as i have a rcd to en61008 that does trip with excessive load!

?

I am trying to answer the original post (quoted) with a reason why

BS EN 61008 RCCBs might trip under excess load.

No one seems to believe me though - as NozSpark said, you have to draw it

 

[Lurch]

I am trying to answer the original post (quoted) with a reason why BS EN 61008 RCCBs might trip under excess load.

No one seems to believe me though - as NozSpark said, you have to draw it

Oh I see, you're answering a different question to the rest of us!

It isn;t tripping under excess load in your example, it's tripping due to a leakage fault, which has nothing to do with excess load.

I see what you're trying to say though.

 

[ADS]

It isn;t tripping under excess load in your example, it's tripping due to a leakage fault, which has nothing to do with excess load.

But the example shows why it may not trip under normal load conditions but could do if the load got excessive

 

[Lurch]

But the example shows why it may not trip under normal load conditions but could do if the load got excessive

No. The RCD will operate when the leakage current due to the fault gets excessive, it doesn't trip because the load is excessive.

Minor pedantry, like I said, I see what you're saying but it's not quite right in the way you word it, but the theory behind is correct.

 

[steptoe]

you are working on a complete coincidence.