Suggestions for better RCD discrimination

[Pretty Mouth]

So, here's a thing. Not conventional for a domestic, and not cheap, but an adjustable RCD could in theory be set for bang-on 300mS @ 30mA / 40mS @ 150mA and be compliant, basically relying on the downstream devices being inherently faster (which we all know they typically are, by a significant margin).

The upstream RCD is probably seeing a lot more leakage than individual downstream RCBOs, because it's protecting the whole installation. It's likely to be closer to it's tripping threshold, so a small amount of leakage on a downstream circuit might push it over the edge, while failing to trip the downstream RCBO.
So the above might work, for some faults, might not.

 

[pc1966]

The upstream RCD is probably seeing a lot more leakage than individual downstream RCBOs, because it's protecting the whole installation. It's likely to be closer to it's tripping threshold, so a small amount of leakage on a downstream circuit might push it over the edge, while failing to trip the downstream RCBO.
So the above might work, for some faults, might not.

Indeed, I would clamp the sub-main to begin with to get an idea of the margin for whatever approach is needed.

But if its is close to 15mA already I don't see any real solution without fixing the initial cable run's need for 30mA additional protection. By time you look at an adjustable solution, etc, the cost is getting seriously large and it might still be a bit flaky!

 

[Rockingit]

Indeed, I would clamp the sub-main to begin with to get an idea of the margin for whatever approach is needed.

But if its is close to 15mA already I don't see any real solution without fixing the initial cable run's need for 30mA additional protection. By time you look at an adjustable solution, etc, the cost is getting seriously large and it might still be a bit flaky!

You're of course absolutely correct and we've had this entire thread without me being on site again yet, however my suspicion is that there's probably no accumulative leakage to speak of at all, what we're dealing with is purely fault current. Downstream of all of this is basically just lighting and typical resistive/inductive stuff in a workshop or two.

As for costs.... the last one I did a couple of years back was around the same £7-800 mark, was a fancy all-in-one Schneider ELCB jobby.

 

[pc1966]

So, here's a thing. Not conventional for a domestic, and not cheap, but an adjustable RCD could in theory be set for bang-on 300mS @ 30mA / 40mS @ 150mA and be compliant, basically relying on the downstream devices being inherently faster (which we all know they typically are, by a significant margin).

I wonder if that is what Scneider's "A-SI" really is? 30mA with a very short delay but still meeting times?

At £300 a bit much to buy one just to play with...

 

[pc1966]

You're of course absolutely correct and we've had this entire thread without me being on site again yet, however my suspicion is that there's probably no accumulative leakage to speak of at all, what we're dealing with is purely fault current. Downstream of all of this is basically just lighting and typical resistive/inductive stuff in a workshop or two.

As for costs.... the last one I did a couple of years back was around the same £7-800 mark, was a fancy all-in-one Schneider ELCB jobby.

Yes, not cheap any how.

I suppose the irony is if this had been found with a 100mA delay RCD and so no "additional protection" it would only get a C3 inspection code!

Assuming no muppet ran the cable through the bathroom of course...

 

[mainline]

The chance of someone putting a nail or something in the cable just at the right spot to receive a shock, and a 100ma S rcd not tripping is highly unlikely, i would say they have more of a chance of being injured with the sudden loss of power.

The chance of a customer paying £1000s to sort out a random tripping problem is even more unlikely.