100ma/30ma rcd for hot tub

[Roberts7286]

Hi

I posted recently about a hot tub supply I was needing advice with. One reply I got was directing me to a video by dave savery. Having watched the video I have a question.

On his install he puts a 100a 100mA rcd and 32A 30mA rcbo in the same enclosure at the house supply end. Meter tails to 100ma and outgoing swa to the rcbo. Can someone explain why this is necessary as I would have thought the 100mA would never trip as the 30mA would have tripped before it.

Tom

 

[buzzlightyear]

100a 100mA rcd and 32A 30mA rcbo in the same enclosure at the house supply end.

why rcd for fault protection and rcbo for over protection .
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.

 

[Roberts7286]

why rcd for fault protection and rcbo for over protection .
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.

Thank you for the straight forward reply. With regard to the delay aspect would this be achieved simply by the difference between 100 and 30 or would the 100 need to be time delayed?

Tom

 

[Dartlec]

Hi

I posted recently about a hot tub supply I was needing advice with. One reply I got was directing me to a video by dave savery. Having watched the video I have a question.

On his install he puts a 100a 100mA rcd and 32A 30mA rcbo in the same enclosure at the house supply end. Meter tails to 100ma and outgoing swa to the rcbo. Can someone explain why this is necessary as I would have thought the 100mA would never trip as the 30mA would have tripped before it.

Tom

Thank you for the straight forward reply. With regard to the delay aspect would this be achieved simply by the difference between 100 and 30 or would the 100 need to be time delayed?

Tom

The 100mA RCD is not strictly necessary to comply with the regulations in this case. He added it to give an extra level of protection in case the 30mA RCD failed. As always, you can go above and beyond the regulations if you do so safely.

With a PME install, a standard main switch and the 30mA RCBO would comply as I understand it. In that case the 30mA RCBO would be for both fault protection for the tub (since it is on TT earthing) and additional protection for the tub. He's added a 100mA as backup, so that is providing fault protection, with the 30mA providing additional protection.

Selectivity can be complicated, and sometimes needs manufacturers literature to exactly work out, but in basic terms, the up stream RCD needs to be less sensitive by a large enough margin to ensure it doesn't trip before the downstream one.

A 100mA one wouldn't normally achieve that, though an S type delayed should.

In older TT systems, the common install was a 100mA S type main switch (for fault protection), with 30mA RCD protection for sockets (additional protection)

If you were designing the perfect system here, it would maybe be more ideal to put a 300mA - but in this case he says in the comments that one of the considerations was easy availability of the parts at a reasonable cost.

Given that there is only one device attached, then selectivity may be a bit moot anyway, since nothing else can be disconnected even if both tripped.

There are definitely varying views on how best to do it, because as yet hot tubs aren't really properly addressed by the Regs as a separate type of install.

 

[buzzlightyear]

Me just go with a rcbo if you can, if not a a Rcd board split and a mcb would be adequate.

 

[Roberts7286]

The 100mA RCD is not strictly necessary to comply with the regulations in this case. He added it to give an extra level of protection in case the 30mA RCD failed. As always, you can go above and beyond the regulations if you do so safely.

With a PME install, a standard main switch and the 30mA RCBO would comply as I understand it. In that case the 30mA RCBO would be for both fault protection for the tub (since it is on TT earthing) and additional protection for the tub. He's added a 100mA as backup, so that is providing fault protection, with the 30mA providing additional protection.

Selectivity can be complicated, and sometimes needs manufacturers literature to exactly work out, but in basic terms, the up stream RCD needs to be less sensitive by a large enough margin to ensure it doesn't trip before the downstream one.

A 100mA one wouldn't normally achieve that, though an S type delayed should.

In older TT systems, the common install was a 100mA S type main switch (for fault protection), with 30mA RCD protection for sockets (additional protection)

If you were designing the perfect system here, it would maybe be more ideal to put a 300mA - but in this case he says in the comments that one of the considerations was easy availability of the parts at a reasonable cost.

Given that there is only one device attached, then selectivity may be a bit moot anyway, since nothing else can be disconnected even if both tripped.

There are definitely varying views on how best to do it, because as yet hot tubs aren't really properly addressed by the Regs as a separate type of install.

So just to clarify if I was to use a 100mA rcd it would be best to use a time delayed one?

Thank you for your response

 

[Roberts7286]

Me just go with a rcbo if you can, if not a a Rcd board split and a mcb would be adequate.

Yeah I've got to split the incoming to an rcd as the existing board is dual rcd so no where to add an rcbo. I'm just trying to get a sound understanding of the install and safety aspect as I hope to use this for my NICEIC assessment. I really appreciate your response. Thank you

 

[Dartlec]

Yeah I've got to split the incoming to an rcd as the existing board is dual rcd so no where to add an rcbo. I'm just trying to get a sound understanding of the install and safety aspect as I hope to use this for my NICEIC assessment. I really appreciate your response. Thank you

If you are already a member, then it may be worth contacting the helpline - they can be quite useful on occasions, and if nothing else it gives you a plausible defence for the method you've chosen. But I would ring with a firm idea of what you plan to do, and ask for confirmation or comments, rather than just ask how to do it...

Is it a PME system, and an external hot tub? Is it on wooden decking or directly on ground?

There's some more advice here, which largely backs up what David mentions.

To comply, I would say splitting the incomers to a new board with RCBO protection for the hot tub cable is perfectly adequate. With a TN-S, no more is needed.

With a PME install, then converting the tub to a TT earth at the isolator end seems like a sensible option, but the RCBO is still sufficient. The extra RCD is merely belt and braces, but if you do it, then an S type delayed RCD is the best option yes.

It's good to have a sound understanding of the options for your assessment, as it will show you have thought carefully about the design if asked.

 

[pc1966]

So just to clarify if I was to use a 100mA rcd it would be best to use a time delayed one?

Yes. The rule-of-thumb for RCD selectivity is each step up is around x3 in current (as above) and an additional 200ms delay per-stage to allow for a hard-fault that has huge currents to be cleared as far down-stream as possible.

Most systems needing a lot of RCD protection (e.g. TT) have 30mA "instant" RCD/RCBO for final circuit additional protection, and 100mA incomer type-S (selective delay) for domestic, or maybe 300mA type-S for larger industry/agriculture boards. Any further up stream typically being adjustable modules set to 300mA/1A or above and 400ms or more delay (typically add-on to incomer MCCB or similar).

 

[Roberts7286]

If you are already a member, then it may be worth contacting the helpline - they can be quite useful on occasions, and if nothing else it gives you a plausible defence for the method you've chosen. But I would ring with a firm idea of what you plan to do, and ask for confirmation or comments, rather than just ask how to do it...

Is it a PME system, and an external hot tub? Is it on wooden decking or directly on ground?

There's some more advice here, which largely backs up what David mentions.

To comply, I would say splitting the incomers to a new board with RCBO protection for the hot tub cable is perfectly adequate. With a TN-S, no more is needed.

With a PME install, then converting the tub to a TT earth at the isolator end seems like a sensible option, but the RCBO is still sufficient. The extra RCD is merely belt and braces, but if you do it, then an S type delayed RCD is the best option yes.

It's good to have a sound understanding of the options for your assessment, as it will show you have thought carefully about the design if asked.

It's for initial assessment to join the scheme. My plan is to split the incoming via Henley block to a small DB Containing either an rcbo and main switch or an rcd and mcb. Swa to isolator with armours connected to pme at new DB. TT at hot tub end. It's going on a patio approx 3 feet from a large conservatory. There's no outside lights etc near it. Plan to install earth rod about 3 metres from tub by garden fence.