100 mA RCD

[dj2102]

What is the purpose of 100 mA RCD ?
Where is requied ?

 

[telectrix]

What is the purpose of 100 mA RCD ?
Where is requied ?

usually on a TT system. there are other applications, e.g. for fault protection where Zs values exceed the max. levels for ADS with fuses or MCBs.

 

[snowhead]

If anyone should know that would be TelecTrix

 

[pc1966]

As said above, for typical domestic application as already said it typically is the incomer on a TT system and normally is the delay (S type) version to downstream 30mA RCD have a chance to clear faults without taking out the whole system. Few TT systems will have a low enough Ze to allow adequate disconnection by even a 6A MCB, hence an RCD is often essential.

Also they are sometimes used for fire protection in agricultural setups even with a TN supply when cables being chewed by rats, etc, are a high risk. There you need a high current to avoid spurious trips, but you want to quickly disconnect on an insulation fault / roasting rat before it starts a fire.

 

[Rockingit]

In my walk of life higher rated RCD's are common place due to massive amounts of cumulative leakage current (common culprit are big LED walls) in perfectly normal safe operation, equally the same in larger commercial or industrial installs so your question needs some context.

 

[Lister1987]

As said above, for typical domestic application as already said it typically is the incomer on a TT system and normally is the delay (S type) version to downstream 30mA RCD have a chance to clear faults without taking out the whole system. Few TT systems will have a low enough Ze to allow adequate disconnection by even a 6A MCB, hence an RCD is often essential.

Also they are sometimes used for fire protection in agricultural setups even with a TN supply when cables being chewed by rats, etc, are a high risk. There you need a high current to avoid spurious trips, but you want to quickly disconnect on an insulation fault / roasting rat before it starts a fire.

I thought 300mA was for fire? 30mA additional, 100mA TT and 300mA for fire?

 

[pc1966]

I think 300mA is the max for fire, as you can also get RCCB style RCD units that can be set up to 5A and longer delay.

For large TT setups you might need 300mA, or maybe even more if a lot of leackage is normal, but what matters is the earth rod Ra is low enough and stable enough so it can disconnect if the worst happens.

But ideally you would plan for suitably coordinated down-stream RCD so the main incomer is unlikely to trip.

 

[Rockingit]

you can also get RCCB style RCD units that can be set up to 5A and longer delay.

Very common scheme for us is that we'll have a 400a/3 primary distro coming straight off a generator and it will have either an adjustable RCCB or an RCM/RCCB setup where both time and current curves can be set digitally as required. This also makes for interesting entertainment sometimes as if you're working in a damp environment for example overnight the cumulative leakage can go very high indeed, easily into 2-3A per phase, then at breakfast you watch them all drop back to something sensible as the sun rises and the air dries!! But...... that requires a whole heap of experience and know-how to keep safe, as I know Dave will agree with me on.

 

[pc1966]

Also to point on in a TN system with low enough Zs values (where the MCB can clear in under 100ms or so) then you can have the delay RCD incomer adequately selective with MCB-only downstream circuits for hard faults to earth (for those circuits that do not need the "additional protection" of a 30mA RCD).

But obviously a 100mA delay (or higher) is not going to provide any form of meaningful additional protection against electric shock as the time/current to trip is dangerously high by human standards.

Also another minor point is an RCD does not limit the current you can get as a shock - it limits the exposure time at currents that could be dangerous. Hence you still need SELV for wet areas, etc, so you don't get a 20ms burst of very high current that causes damage, and not just thinking a 10mA RCD on 240V is going to be magically safe.

 

[Burtones]

I thought you don't have to have a 100mA RCD on TT if you have dual 30mA RCD consumer unit or full RCBO board? I've only ever seen an upfront RCD on older TT installs that don't have RCD protection in the consumer unit.

If I'm right, are there any cases where you would still want to install an s-type 100mA upfront in addition to 30mA on all circuits?

 

[pc1966]

If all of your circuits are on RCD feeds, or from RCBOs, then you don't need the up-front 100mA delay RCD.

But then you are dependent on the RCD for clearing a fault, so you have some relatively complex electronics as a single point of failure. Even though there is a test button and labels to say it should be tested every 6 months, it won't be.

So if it were my own house I would still have the up-front RCD in a TT system.

Another factor is the use of SPD. They should be before any normal 30mA RCD as generally such RCD are not rated to survive multi-kA fault currents (as lightning will induce) but usually delay RCD incomers are. Now it might be your SPD is something that you feel will not need RCD protection, but should it fail on a TT setup in any sort of short-circuit mode then the earth/CPC will remain live unless there is an up-front RCD as the supply fuse will not be blown by the usual Ra range of impedances.

 

[UKPN]

I thought 300mA was for fire? 30mA additional, 100mA TT and 300mA for fire?

I think a 100TD S type is best for up front TT. A 100ma has a max of 300ms discon, a 100TD S type has a max of 500 ms discon.

 

[Mark Wright]

that is interesting because why would you use 100mA when you could put 30mA, except type S for distribution circuit.

 

[pc1966]

that is interesting because why would you use 100mA when you could put 30mA, except type S for distribution circuit.

It depends on what you are protecting, and how much it is expected to leak "normally" versus "faulty".

30mA is basically for human protection, though you get lower values.

But large systems, especially electronics, can leak more than that due to all of the capacitance L-E so you might need 100mA, or more, to provide protection against 'soft' faults that are not your MCB Zs-level of fault current (for example water getting in to a motor or junction box), or if the supply Zs is too high (classic TT case). Often such 'soft' faults quickly develop in to hard faults that can damage other things when they do go bang.

If you need selectivity then you have to have both a delay aspect and a 3:1 or so difference in trip current. For example, a 100mA device could trip anywhere between 50-100mA and you want some margin over a 30mA one, etc, for small persistent fault currents.

But if you get a hard L-E short then all cascaded RCD will have their current trip thresholds exceeded, so you need them to fire is sequence so the one closest to the fault fires before the one above it decided to act, etc. Much the same idea as cascaded OCPD really.