PowerBreaker RCD fused spur instead of standard fused spur switch?

[Mikey]

Please can anyone advise if i can use a PowerBreaker RCD fused spur instead of
a standard fused spur switch.
I understand that my RCD fuse box will cut incase of any fault but would like
to replace the standard fused spur switch with the Powerbreaker RCD
(Greenbrook) which feeds my outside pond pump and outside socket. In which
case if either fail it will just cut out at the Powerbreaker RCD instead of
the fusebox. Or must I still combine a standard fused spur switch with the RCD
powerbreaker.

--
for full context, visit https://www.homeownershub.com/uk-diy/powerbreaker-rcd-fused-spur-instead-of-standard-fused-spur-s-1421533-.htm

[spuorg...@gowanhill.com]

On Wednesday, 8 April 2020 16:14:04 UTC+1, Mikey wrote:
> I understand that my RCD fuse box will cut incase of any fault but would like
> to replace the standard fused spur switch with the Powerbreaker RCD
> (Greenbrook) which feeds my outside pond pump and outside socket. In which
> case if either fail it will just cut out at the Powerbreaker RCD instead of
> the fusebox.

No, it won't. There is no discrimination between RCDs and either or both will cut out in the event of an earth fault.

Owain

[John Rumm]

On 08/04/2020 16:14, Mikey wrote:
> Please can anyone advise if i can use a PowerBreaker RCD fused spur
> instead of
> a standard fused spur switch.
> I understand that my RCD fuse box will cut incase of any fault but would
> like
> to replace the standard fused spur switch with the Powerbreaker RCD
> (Greenbrook) which feeds my outside pond pump and outside socket. In which
> case if either fail it will just cut out at the Powerbreaker RCD instead of
> the fusebox. Or must I still combine a standard fused spur switch with
> the RCD
> powerbreaker.

Unfortunately if there is excess earth leakage, both RCDs will see it
and either or both may trip. Even if they have different trip
thresholds, if there is enough leakage to trip the upstream one you will
still see one or both trip.

This is a problem known as "discrimination" i.e. the practice of
ensuring that only the protective device for the circuit (or part of)
close to where the fault condition exists trips, and not ones
responsible for other parts of the installation.

The only way to ensure you have discrimination with cascaded RCDs is to
use a type with a built in time delay[1] in the upstream part of the
circuit.


[1] Usually called a Type S (S for "selective")

http://wiki.diyfaq.org.uk/index.php/RCD#System_design_using_RCDs




--
Cheers,

John.

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[ARW]

On 08/04/2020 17:20, John Rumm wrote:

> The only way to ensure you have discrimination with cascaded RCDs is to
> use a type with a built in time delay[1] in the upstream part of the
> circuit.
>
>
> [1] Usually called a Type S (S for "selective")
>

And is probably going to be very difficult for the OP to install and
meet the regs

--
Adam

[The Natural Philosopher]

That entirely depends on what values and types of RCD are installed


--
"Socialist governments traditionally do make a financial mess. They
always run out of other people's money. It's quite a characteristic of them"

Margaret Thatcher

[John Rumm]

Indeed, with the 18th edition requirements that all cable runs have RCD
protection unless they are buried >= 50mm from the surface, or be
actually wired visibly on the surface, or are protected by earthed
metallic screening (conduit, armour etc), its complicated to get a non
RCD protected feed to a place where you can sensibly stick your RCD spur.

A split load CU, with a non RCD section, feeding a MCB feeding a
dedicated radial circuit wired in earthshield or SWA etc to the spur
position, and then RCD connection unit at that point.

[ARW]

On 08/04/2020 19:32, John Rumm wrote:
> On 08/04/2020 18:08, ARW wrote:
>> On 08/04/2020 17:20, John Rumm wrote:
>>
>>> The only way to ensure you have discrimination with cascaded RCDs is
>>> to use a type with a built in time delay[1] in the upstream part of
>>> the circuit.
>>>
>>>
>>> [1] Usually called a Type S (S for "selective")
>>>
>>
>>
>> And is probably going to be very difficult for the OP to install and
>> meet the regs
>
> Indeed, with the 18th edition requirements that all cable runs have RCD
> protection unless they are buried >= 50mm from the surface, or be
> actually wired  visibly on the surface, or are protected by earthed
> metallic screening (conduit, armour etc), its complicated to get a non
> RCD protected feed to a place where you can sensibly stick your RCD spur.
>
> A split load CU, with a non RCD section, feeding a MCB feeding a
> dedicated radial circuit wired in earthshield or SWA etc to the spur
> position, and then RCD connection unit at that point.
>
>

Far too over engineered for my liking John:-)

If it's a radial from the CU then you would just fit a RCBO in the CU
and not bother with the SWA or earthshield [1] and then just use T&E up
to the final connection to the pond.

[1] Do they still sell it? Did they ever sell any? Was it ever rated for
6mm with a 32A MCB? I think I once saw some.



--
Adam

[John Rumm]

Well yes, but it does skip the RCD at the point of use. I was assuming
here that the OPs desire was to have any trip occur in a place that does
not require a schlep back to the CU to reset it.

> [1] Do they still sell it? Did they ever sell any? Was it ever rated for
> 6mm with a 32A MCB? I think I once saw some.

Not sure I have ever seen any (earthshield that is) in real life...

[tabb...@gmail.com]

On Wednesday, 8 April 2020 16:14:04 UTC+1, Mikey wrote:

> Please can anyone advise if i can use a PowerBreaker RCD fused spur instead of
> a standard fused spur switch.
> I understand that my RCD fuse box will cut incase of any fault but would like
> to replace the standard fused spur switch with the Powerbreaker RCD
> (Greenbrook) which feeds my outside pond pump and outside socket. In which
> case if either fail it will just cut out at the Powerbreaker RCD instead of
> the fusebox. Or must I still combine a standard fused spur switch with the RCD
> powerbreaker.

I'm not aware of anything preventing you using an RCD fused spur on its own. As said there's no knowing which will trip if a leakage event occurs, but if you've already got the RCD spur I guess that beats 100% chance of it being the CU one to trip. If you were thinking of buying one I doubt it's worth it.


NT

[Mikey]

replying to John Rumm, Mikey wrote:
Many thanks everyone.
It seems that it is not worth doing then as like you say either or both could
trip anyway.
I thought as the PowerBreaker RCD fused spur would be the foremost directly
linked switch to the sockets that it would trip first instead of the fusebox.
The powerbreaker is a passive 30mA but not sure what mA the fusebox would trip
at.

[Mikey]

replying to tabbypurr, Mikey wrote:
Thanks for your reply.
I did had already purchased the Greenbrook 10mA Powerbreaker.
I guess I could try it and see which possibly trips first.
Is it possibly double safety in the fact that i would have two break points in
case of failure?

[John Rumm]

On 09/04/2020 15:44, Mikey wrote:
> replying to John Rumm, Mikey wrote:
> Many thanks everyone.
> It seems that it is not worth doing then as like you say either or both
> could
> trip anyway.
> I thought as the PowerBreaker RCD fused spur would be the foremost directly
> linked switch to the sockets that it would trip first instead of the
> fusebox. The powerbreaker is a passive 30mA but not sure what mA the
> fusebox would trip

A typically RCD in the CU would also be 30mA trip. However since the RCD
has no control over the magnitude of the leakage current, and they are
both seeing the same leakage current, there is not way of predicting
what will trip in most cases.

[John Rumm]

On 09/04/2020 22:14, Mikey wrote:
> replying to tabbypurr, Mikey wrote:
> Thanks for your reply. I did had already purchased the Greenbrook 10mA
> Powerbreaker. I guess I could try it and see which possibly trips first.
> Is it possibly double safety in the fact that i would have two break
> points in
> case of failure?

No, just double the odds of getting a failed RCD or nuisance trip :-)

If you have a 10mA trip device downstream of a 30mA trip device, then
there is a narrow window of leakage currents (anything from ~10 to
~20mA) that would trip the "smaller" device in preference.

[Scott]

When I upgraded the consumer unit, I asked for RCBOs instead of RCDs
(eight of them). I gave the computer a circuit of its own. Same for
alarm system. If you gave the outside electrics a separate RCBO, this
would achieve the purpose.

The electrician was shocked (excuse the pun!) when I suggested this as
he said it would be far too expensive but when he priced it up it was
much cheaper than he expected and he agreed it was the 'way to go'.

[Mike Clarke]

On 09/04/2020 23:36, John Rumm wrote:
> On 09/04/2020 15:44, Mikey wrote:
>> replying to John Rumm, Mikey wrote:
>> Many thanks everyone.
>> It seems that it is not worth doing then as like you say either or
>> both could
>> trip anyway.
>> I thought as the PowerBreaker RCD fused spur would be the foremost
>> directly
>> linked switch to the sockets that it would trip first instead of the
>> fusebox. The powerbreaker is a passive 30mA but not sure what mA the
>> fusebox would trip
>
> A typically RCD in the CU would also be 30mA trip. However since the RCD
> has no control over the magnitude of the leakage current, and they are
> both seeing the same leakage current, there is not way of predicting
> what will trip in most cases.

Given that 30mA is a nominal level and tolerances will be different for
each it's pot luck which would trip first. But if they both trip at
exactly the same current the RCD in the CU could be more likely to trip
first since it would also 'see' any small background leakage elsewhere
in the circuit(s) which would be in addition to what the local RCD sees

[Mikey]

replying to John Rumm, Mikey wrote:

Thanks John,
Do you think I should give it a go and replace the standard fused spur switch
with the 10mA passive powerbreaker or not bother?

[John Rumm]

On 10/04/2020 13:44, Mikey wrote:
> replying to John Rumm, Mikey wrote:
> Thanks John, Do you think I should give it a go and replace the standard
> fused spur switch
> with the 10mA passive powerbreaker or not bother?

I suppose that depends on what are you hoping to achieve?

Will it be "safer", no - the 30mA trip device (if that is what you
have[1]) will offer adequate shock protection.

If you are dealing with a circuit that trips often and would like to
save a walk back to the CU on some occasions, the the additional one
*might* do that *sometimes*. However you would likely be better off
fixing the case of the frequent trips.


[1] Normally a RCD in a CU is a 30mA trip device, and that will protect
people against the more egregious effects of shock. Some CUs (notably
those complying with the 16th edition of the wiring regs) and using TT
earthing (i.e. those with no earth provision from the electrical supply)
may have 100mA trip RCDs covering all circuits. These are for equipment
and fire protection, but do not offer adequate protection for people
from electrocution.

[tabb...@gmail.com]

On Thursday, 9 April 2020 22:14:04 UTC+1, Mikey wrote:
> replying to tabbypurr, Mikey wrote:

> Thanks for your reply.
> I did had already purchased the Greenbrook 10mA Powerbreaker.
> I guess I could try it and see which possibly trips first.

alerady been explained why this is pointless

> Is it possibly double safety in the fact that i would have two break points in
> case of failure?

no. why was already explained.


NT

[Mikey]

replying to John Rumm, Mikey wrote:

Thanks John.
Removed the cable with the earth corrosion showing and all good now.

[John Rumm]

On 14/04/2020 15:14, Mikey wrote:

> replying to John Rumm, Mikey wrote:
> Thanks John. Removed the cable with the earth corrosion showing and all
> good now.

So probably compromised insulation in the cable giving it a low
insulation resistance.

(I have a three core SWA in my garden that is showing low IR on one core
for no obvious reason - I had to abandon that core in the end.
Fortunately it was a lighting circuit with PIR control, and the extra
core was just used as an "always on" override)