RCD on the Consumer Unit and also remotely...

[xelaris]

I'm just contemplating the idea of replacing an old Hager consumer unit (6 ways...no RCD)....A few appliances I've installed have a dedicated RCD (underfloor heating cable and steam shower) - note: the RCDs are not in the Consumer Unit but next the appliances in question (it should be obvious from the title).
What are the benefits or disadvantages (if any) if in a non distant future I decide to update the CU with a 17th edition complete with RCDs?
Will both RCD from the appliance and from the CU trigger at the same time? If the appliance being affected by a fault has a dedicated RCD would this mean that it would be shut down while the rest of the circuit will remain live?

I'd like the idea that there might still be some benefits in having some remote RCD and on the consumer unit - not just an overkill or worst still an over-sensitive system....

Any idea?

 

[Taylortwocities]

There is nothing dangerous about having two RCDs in a particular circuit. The problem is that (unless they are special time-delayed RCDs) you can never predict which RCD is going to trip in the event of a fault.

Usually it will be the one with the faster trip time, or it can be something as esoteric as where the AC sinewave is at the time of the fault. Sometimes both will trip. So there's no real benefit.

I would remove the RCDs and replace them with standard FCUs or switches, depending on their type and function.

 

[xelaris]

There is nothing dangerous about having two RCDs in a particular circuit. The problem is that (unless they are special time-delayed RCDs) you can never predict which RCD is going to trip in the event of a fault.

Usually it will be the one with the faster trip time, or it can be something as esoteric as where the AC sinewave is at the time of the fault. Sometimes both will trip. So there's no real benefit.

I would remove the RCDs and replace them with standard FCUs or switches, depending on their type and function.

Why changing a remote RCD for a FCU? In a way the RCD can act as a manual switch anyway...

Anyhow, this unpredictability bothers me....
In the event of a fault in the appliance with dedicated RCD we'll be presented with 3 possible outcomes:
1) both RCDs will trigger
2) only the RCD on the CU will trigger
3) only the RCD on faulty appliance will trigger
Out of the 3 only the 3rd is useful - the others are just extra walking and timewasting....1/3 is no good to me.

Then, what the hell is going to happen when the fault is not in the appliance with dedicated RCD?

1/3 x 1/3 = 1/9 if my maths are right (more no good)

I can see problems - e.g. forgetting to switch on the steam shower's RCD cos' we only bothered with the one on the CU...then of course we'll find the hard and cold truth when we go for our shower the morning after!

Rrrrrrr

 

[JohnW2]

Anyhow, this unpredictability bothers me....
In the event of a fault in the appliance with dedicated RCD we'll be presented with 3 possible outcomes:
1) both RCDs will trigger
2) only the RCD on the CU will trigger
3) only the RCD on faulty appliance will trigger
Out of the 3 only the 3rd is useful ...

Well, they are all useful - since in every case, at least one RCD has tripped, in response to the fault, thereby disconnecting the electricity.

- the others are just extra walking and timewasting....1/3 is no good to me.

How often do you expect an RCD to trip? I've lived with about 10 RCDs for many years, and could probably count the number of times that ones has tripped (other than due to some silliness on my part) on the fingers of my hands.

Then, what the hell is going to happen when the fault is not in the appliance with dedicated RCD?

The one in the CU should trip.

I can see problems - e.g. forgetting to switch on the steam shower's RCD cos' we only bothered with the one on the CU...then of course we'll find the hard and cold truth when we go for our shower the morning after!

As above, how often do you expect either of them to trip?

Kind Regards, John

 

[AndyPRK]

Having two can be useful if both trip as it will tell you which circuit caused it and allow the rest of the circuits to be repowered easily.

 

[JohnW2]

Having two can be useful if both trip as it will tell you which circuit caused it ...

... so long as it's the circuit which does have two!

Kind Regards, John

 

[xelaris]

Anyhow, this unpredictability bothers me....
In the event of a fault in the appliance with dedicated RCD we'll be presented with 3 possible outcomes:
1) both RCDs will trigger
2) only the RCD on the CU will trigger
3) only the RCD on faulty appliance will trigger
Out of the 3 only the 3rd is useful ...

Well, they are all useful - since in every case, at least one RCD has tripped, in response to the fault, thereby disconnecting the electricity.

- the others are just extra walking and timewasting....1/3 is no good to me.

How often do you expect an RCD to trip? I've lived with about 10 RCDs for many years, and could probably count the number of times that ones has tripped (other than due to some silliness on my part) on the fingers of my hands.

Then, what the hell is going to happen when the fault is not in the appliance with dedicated RCD?

The one in the CU should trip.

I can see problems - e.g. forgetting to switch on the steam shower's RCD cos' we only bothered with the one on the CU...then of course we'll find the hard and cold truth when we go for our shower the morning after!

As above, how often do you expect either of them to trip?

Kind Regards, John

if you ask me....I don't expect any RCD to trip cos' I expect the electrical system to be perfect...(in fact all these RCDs and safety stuff seems more and more an headless chicken for keeping the electricians happy).
Then - in reality it happened twice in 9 months because of a faulty hair clipper which triggered the RCD of the Underfloor Heating cable!! If there was a second RCD in the CU I'd checked that in the first place and totally ignore the one of the UFH cable...
Anyhow....I was trying to evaluate the pro/against of the present system (made of an old CU and a few remote RCDs) versus an updated CU (17th edition) - I thought it might be less work leaving the present remote RCDs where they are and going for the better CU models (with more ways) to use some dedicated circuits for the appliances connected to these (e.g. a radial for the shower and another for the UFH and maybe another 1 or 2 for other critical areas).
I hoped there where some elements fo predictability - e.g. if the faulty item is the steam shower I expect only the dedicated RCD to trip - but this might not be the case....so it's total randomness - unless one starts reading the small prints about triggering reaction times...(while hoping the declared specs are right).
Anyhow....more dedicate radial circuits might be the best solution - just a little bit pricier.

 

[securespark]

(in fact all these RCDs and safety stuff seems more and more an headless chicken for keeping the electricians happy).

Right.

You have quite obviously never seen (or smelt) the effect of electricity on human flesh.

 

[securespark]

Duplicate.

 

[JohnW2]

I hoped there where some elements fo predictability - e.g. if the faulty item is the steam shower I expect only the dedicated RCD to trip - but this might not be the case....so it's total randomness - unless one starts reading the small prints about triggering reaction times...(while hoping the declared specs are right).

Quite so. In fact, if there are two 30mA standard RCDs in the same circuit, no amount of reading or hoping is going to tell you which (or both) will trip in the case of a fault.

Anyhow....more dedicate radial circuits might be the best solution - just a little bit pricier.

Yep - it sounds as if the only thing which would keep you happy would be lots of circuits, ideally all dedicated, each with it's own RCBO. That is do-able.

Kind Regards, John

 

[bernardgreen]

if you ask me....I don't expect any RCD to trip cos' I expect the electrical system to be perfect...(in fact all these RCDs and safety stuff seems more and more an headless chicken for keeping the electricians happy).

It is when some headless DIYer puts a metal shelf bracket on the wall and one of the screws goes into the live conductor in a buried cable that RCDs can be very useful in protecting people from serious electric shock from an electrically live metal shelf bracket.

 

[xelaris]

(in fact all these RCDs and safety stuff seems more and more an headless chicken for keeping the electricians happy).

Right.

You have quite obviously never seen (or smelt) the effect of electricity on human flesh.

we'll triplicate this to the guy who asked me something like "how many times do you expect the RCD to trip...."
Then....no I haven't seen (or smelt) the effect of electricity on human flesh...have you? was that something to do with a job you completed yourself?

Have all these RCD and extra safety measures have any effect on safety? If so can you post a link? Then, we'll post that to the same guy who asked me "how many times do you expect the RCD to trip" Doh...

Maybe....I repeat "maybe" all these security and extra security stuff create more troubles rather than solving them...
What next? after the RCD story subsides and all of us have complied with the new regulations? What the Leccy lobby has in store to get us buying new stuff and services? (totally redundant and an overkill in the majority).

I'll start a new lobby for a 12v DC system for all....

 

[AndyPRK]

Why aren't more rcd sockets 10ma rather than 30ma?

Would they cause too many trips with garden tools?

 

[Taylortwocities]

if you ask me....I don't expect any RCD to trip cos' I expect the electrical system to be perfect...(in fact all these RCDs and safety stuff seems more and more an headless chicken for keeping the electricians happy).

It is when some headless DIYer puts a metal shelf bracket on the wall and one of the screws goes into the live conductor in a buried cable that RCDs can be very useful in protecting people from serious electric shock from an electrically live metal shelf bracket.

Absolutely, xelaris. This is from a while ago, but it put extra emphasis on to the addition of RCDs on all circuits, which was incorporated into the Wiring Regulations. It also gave a boost to the addition of Electrical Safety into Building Regulations (Part P).
http://news.bbc.co.uk/1/hi/uk_politics/3735928.stm

If RCDs had been installed on the final circuits then Mary Wherry would still be alive.

PS I still really do not uinderstand why th 'powers' have removed kitchens from the notification process. In my experience, they are the worst place in many houses due to unaware trades, or DIYers "doing up" the kitchen.

 

[JohnW2]

Why aren't more rcd sockets 10ma rather than 30ma?

An interesting question. The RCD will, of course, not in any way 'limit' the current through a victim (only the duration), and I imagine the great majority of potentially serious electric shocks are probably >30mA (i.e. impedance of path through person less than ~7.7k&#937 - so if a 10mA RCD would operate, then so would a 30mA one.

Kind Regards, John