Wiring log cabins

Bruthal

Sir Liamalot wrote: »

Not if you cascade the trip thresholds.

100ma in series with a 30ma. 5 amp fault. Which one goes?

Zarco

Bruthal wrote: »

100ma in series with a 30ma. 5 amp fault. Which one goes?

They actually have it in the rules that for selectivity you need the time delay or twice the rated trip current upstream

I'm suspicious about the trip ratings providing discrimination

It's all academic anyway until some idiot decides one is needed upstream

That's leakage current I'm on about there

Bruthal

Bruthal wrote: »

100ma in series with a 30ma. 5 amp fault. Which one goes?

Not having one at the incomer and slicing the shed feeder with a mini-digger which one goes?....

I'd rather the cascade for local nuisance tripping.

Tuco88

By supplying a shed with an upstream RCBO. Suppose you would also be gaining additional fire protection if the cable is passing through a portion of the dwelling like the attic.

Nuisance tripping... It was always a fault for me most of the time, or poor design on circuits with equipment with some form of leakage current like PCs.

The other side from this forum shower RCBOs look like they cause nuisance tripping.

Anyway to save the legs, Id supply from an MCB. There is not much in a domestic dwelling now not rcd protected Fire alarm,Cooker, shed depending, the odd fixed appliance?

2011

Sir Liamalot wrote: »

Not having one at the incomer and slicing the shed feeder with a mini-digger which one goes?....

I'd rather the cascade for local nuisance tripping.

I wouldn’t be concerned for the mini digger driver to be honest.
RCD’s are a great invention but I don’t see the benefit of installing them in series in the way suggested. Just like I wouldn’t advocate installing one upstream of the main distribution board in a domestic installation.

2011

Tuco88 wrote: »

By supplying a shed with an upstream RCBO. Suppose you would also be gaining additional fire protection if the cable is passing through a portion of the dwelling like the attic.

How? Only if the heat is due to an earth fault that was not of sufficient magnitude to trip the MCB. I wouldn’t like to rely on an RCD to prevent a fire. There are far better ways to prevent a fire.

Anyway to save the legs, Id supply from an MCB. There is not much in a domestic dwelling now not rcd protected Fire alarm,Cooker, shed depending, the odd fixed appliance?

Apart form saving the legs it’s best practice.
That’s why we do it that way in industrial installations.

Bruthal

Sir Liamalot wrote: »

Not having one at the incomer and slicing the shed feeder with a mini-digger which one goes?....

I'd rather the cascade for local nuisance tripping.

I must be losing it. Never even thought of the mini digger scenario:pac:

We often cascade RCBO distro in events...back when they were a thing. It's more reliable than not. Many time it's stopped caterers taking out the PA. Bigger gigs have isolated supplies.

2011

Sir Liamalot wrote: »

We often cascade RCBO distro in events...back when they were a thing. It's more reliable than not. Many time it's stopped caterers taking out the PA. Bigger gigs have isolated supplies.

These are temporary installations attended by the general public, as such they have a very different risk profile which can justify a different approach. The OP is discussing something quite different.

I agree to differ 2011. It has merits. I go belt agus braces. I understand you are usually right and it's not a reg requirement.
I also put RBCOs on vessels that have a (usually defective or absent) shore power flavour from the supplier... both at 30mA
The benefits outweigh the cons imho.

Regarding the "it brings nothing to the party" isolation is almost guaranteed albeit with a bittova walk sometimes. System RCD failure rate goes from 4% to decimal with two series.

Tuco88

2011 wrote: »

How? Only if the heat is due to an earth fault that was not of sufficient magnitude to trip the MCB. I wouldn’t like to rely on an RCD to prevent a fire. There are far better ways to prevent a fire.



Apart form saving the legs it’s best practice.
That’s why we do it that way in industrial installations.

You just answered the question. Is that not one of the uses for 100/300mA RCDs is some form of fire protection. You'd take it 30mA would do the same but not sure if 6watts is enough to start a fire. Im not arguing vs smoke detector or vesta system in an attic. Just a possible advantage or RCD protected circuits.

Risteard81

Sir Liamalot wrote: »

Not if you cascade the trip thresholds.

That will only work with an RCD with a time delay (selective (S-type)).

Incidentally the likes of caravan hookups deliberately have two RCDs in series - one for the pitch and one for the caravan. Marinas are another example of this.

Selectivity (or discrimination in old money) is only an absolute requirement where necessary for safety - i.e. where disconnection would create a hazard. Obviously it's not necessarily good design to have several similar RCDs in series however.

Risteard81

Risteard81 wrote: »

That will only work with an RCD with a time delay (selective (S-type)).

Can you elaborate Risteard?


If the incomer is protected at 100mA and the load local is 30mA where the fault current is 40mA from noisey power supplies or unsanctioned floating motors why does this not work?

Edit... Ah I see you say similar RCDs...I did not mean this. That's not a cascade by my definition. Apologies for the confusion.

2011

Sir Liamalot wrote: »

I agree to differ 2011. It has merits. I go belt agus braces. I understand you are usually right and it's not a reg requirement.
I also put RBCOs on vessels that have a (usually defective or absent) shore power flavour from the supplier... both at 30mA
The benefits outweigh the cons imho.

Regarding the "it brings nothing to the party" isolation is almost guaranteed albeit with a bittova walk sometimes. System RCD failure rate goes from 4% to decimal with two series.

None of the installations you have mentioned (events, boats) are comparable with what is mentioned in the OP. In these different scenarios I agree. You are quoting what I said for a log cabin and assuming that I would be applying the same logic to a boat or event.

Regardless of whether you or I would put RCDs in series or not for outbuildings this is not normal practice or a requirement of the rules.

I have worked on drilling platforms and have extensive experience with a particular type of marine application. When doing this work we did lots of things that would never be applied to log cabin supplies in domestic installations (such as insulation monitoring relays).

2011

Tuco88 wrote: »

You just answered the question. Is that not one of the uses for 100/300mA RCDs is some form of fire protection. You'd take it 30mA would do the same but not sure if 6watts is enough to start a fire. Im not arguing vs smoke detector or vesta system in an attic. Just a possible advantage or RCD protected circuits.

I wouldn't be suggesting to a customer / client that you are going to provide additional fire protection by installing an RCD on a domestic circuit. I agree that it is possible that an RCD could prevent a fire, but I can think of far better ways if this was a genuine concern. One way would be to install AFDDs.

You work in pharma (so do I at the moment). Look around you, they have very deep pockets and take fire prevention very seriously. Even in this type of environment with flammable solvents sub boards are not fed from RCDs.

In Atex areas we generally don't use RCD's to mitigate the risk of an explosion on final circuits for fixed appliances/ instruments. One notable exception would be heat tracing circuits, but there is a different reason for this.

The mitigation measure has to fit the risk profile.

My workshop is on the house RCD. Then again it doesn't have a Subdisto.
Looks like it was a stop gap. They wired it on 10mm² 20A MCB and it's only about 20meters of cable

Tuco88

2011 wrote: »

I wouldn't be suggesting to a customer / client that you are going to provide additional fire protection by installing an RCD on a domestic circuit. I agree that it is possible that an RCD could prevent a fire, but I can think of far better ways if this was a genuine concern. One way would be to install AFDDs.

You work in pharma (so do I at the moment). Look around you, they have very deep pockets and take fire prevention very seriously. Even in this type of environment with flammable solvents sub boards are not fed from RCDs.

In Atex areas we generally don't use RCD's to mitigate the risk of an explosion on final circuits for fixed appliances/ instruments. One notable exception would be heat tracing circuits, but there is a different reason for this.

The mitigation measure has to fit the risk profile.

Ageed, I would also not be trying to push rcd devices as a FP. But if the op was to install a 300mA s-type is the just a forn of advantage he night get. The ATEX is another ball game, with I.S circuits and more specific the design and zone classification of the equipment also agree on that one.

2011

Tuco88 wrote: »

Ageed, I would also not be trying to push rcd devices as a FP. But if the op was to install a 300mA s-type is the just a forn of advantage he night get. The ATEX is another ball game, with I.S circuits and more specific the design and zone classification of the equipment also agree on that one.

Yes ATEX is a different animal and highly specialized. I have been working in this area for a very long time and still feel I have a lot to learn.