RCD for professional sunbed circuit installation (edited.)

spike47 · 13 Feb 2021

Hi

I have a Sunbed that needs wiring in, it has a load of 6700W , am thinking of getting somerthing like the following item ( Pic attached for 2 types ) been told though that this needs to have a type "C" MCB and the RCD needs to have a trip of 100mA as apposed to the normal 30mA trip, they reckon it is like a electric motor when starting up ! .
Can anyone confirm if this is the case, not easy to find 45A 100mA .

cheers
Spike

Taylortwocities · 13 Feb 2021

spike47 said:
not easy to find 45A 100mA

Therefore use a higher rated 100mA RCD, 63A or 80A perhaps, with a 40A MCB after it.
The ampere rating of an RCD refers to the rating of the contact current, not any form of tripping capability.

333rocky333 · 13 Feb 2021

If the wiring is installed in such a way that it needs 30ma protection, then surely its not possible to wire it to a 100ma Rcd,
Even if you fed that remote enclosure and put in a 100ma, fed from the 45 amp Mcb, there would be no point as it appears the 45a Mcb is fed off the adjacent 30ma trip

JohnW2 · 13 Feb 2021

spike47 said:
I have a Sunbed that needs wiring in, it has a load of 6700W , am thinking of getting somerthing like the following item ( Pic attached for 2 types ) been told though that this needs to have a type "C" MCB and the RCD needs to have a trip of 100mA as apposed to the normal 30mA trip, they reckon it is like a electric motor when starting up ! .

Some of that doesn't make too much sense.

Even if it "starts up like an electric motor", that has no relevance in terms of the residual current trip threshold - since such trips only occur in response to leakages of current to earth. I also wonder what they perceive as being the point/reason for the residual current protection (i.e. an RCBO, rather than an MCB), given that 100 mA trip threshold is not usually considered low enough to protect human life.

I also wonder where all that 6,700 W is going - does it perhaps involve 'heating' as well as the 'sunbed' function? I would have thought that anything approaching 6.7 kW of UV would be incredibly dangerous to human beings!

Also, if you did need a Type C MCB (or RCBO) then unless the total length of the cable run (from sunbed all the way back to your CU) were very short, you might need pretty hefty cable.

Kind Regards, John

EFLImpudence · 13 Feb 2021

spike47 said:
been told though that this needs ...

Who told you?

flameport · 13 Feb 2021

spike47 said:
I have a Sunbed that needs wiring in

The installation of a sunbed is not DIY, and is far outside the scope of this or any other forum.

spike47 · 13 Feb 2021

EFLImpudence said:
Who told you?

endecotp · 13 Feb 2021

Relevant bit from the very end of that “tech info”pdf:

RCD’s trip with an earth leakage of 30mA which is sufficiently low enough to protect against electric shock, and is the industry standard size of RCD. But what is unusual about sunbeds is that the choke (ballast) inside that supplies each lamp, by its very nature, has a natural earth leakage of around 0.75mA.. This is inherent in its design, as a copper coil surrounding a soft iron core. Hence a 40 tube sunbed can easily have a total earth leakage of 30mA, and a 50 tube sunbed almost certainly would.

spike47 · 13 Feb 2021

JohnW2 said:
Some of that doesn't make too much sense.

Even if it "starts up like an electric motor", that has no relevance in terms of the residual current trip threshold - since such trips only occur in response to leakages of current to earth. I also wonder what they perceive as being the point/reason for the residual current protection (i.e. an RCBO, rather than an MCB), given that 100 mA trip threshold is not usually considered low enough to protect human life.

I also wonder where all that 6,700 W is going - does it perhaps involve 'heating' as well as the 'sunbed' function? I would have thought that anything approaching 6.7 kW of UV would be incredibly dangerous to human beings!

Also, if you did need a Type C MCB (or RCBO) then unless the total length of the cable run (from sunbed all the way back to your CU) were very short, you might need pretty hefty cable.

Kind Regards, John

Going to use 6mm2

spike47 · 13 Feb 2021

endecotp said:
Relevant bit from the very end of that “tech info”pdf:

RCD’s trip with an earth leakage of 30mA which is sufficiently low enough to protect against electric shock, and is the industry standard size of RCD. But what is unusual about sunbeds is that the choke (ballast) inside that supplies each lamp, by its very nature, has a natural earth leakage of around 0.75mA.. This is inherent in its design, as a copper coil surrounding a soft iron core. Hence a 40 tube sunbed can easily have a total earth leakage of 30mA, and a 50 tube sunbed almost certainly would.

Mine has 40 tubes

JohnW2 · 13 Feb 2021

endecotp said:
Relevant bit from the very end of that “tech info”pdf:

RCD’s trip with an earth leakage of 30mA which is sufficiently low enough to protect against electric shock, and is the industry standard size of RCD. But what is unusual about sunbeds is that the choke (ballast) inside that supplies each lamp, by its very nature, has a natural earth leakage of around 0.75mA.. This is inherent in its design, as a copper coil surrounding a soft iron core. Hence a 40 tube sunbed can easily have a total earth leakage of 30mA, and a 50 tube sunbed almost certainly would.

I have to say that's a new one on me. Capacitive leakage to earth is straightforward enough, but I've not come across (and struggle a bit to understand) 'inductive leakage'. On the face of it, that concept would apply to any wirewound component (motor, transformer etc.) would suffer from the same issue. Can someone educate me?

I also still wonder where all that 6,700 W is going. As I said, unless there is some ('conventional') heating involved, it sounds like a frightening amount of energy to be turning into UV!

Kind Regards, John

spike47 · 13 Feb 2021

JohnW2 said:
I have to say that's a new one on me. Capacitive leakage to earth is straightforward enough, but I've not come across (and struggle a bit to understand) 'inductive leakage'. On the face of it, that concept would apply to any wirewound component (motor, transformer etc.) would suffer from the same issue. Can someone educate me?

I also still wonder where all that 6,700 W is going. As I said, unless there is some ('conventional') heating involved, it sounds like a frightening amount of energy to be turning into UV!

Kind Regards, John

There is also quite a large fan on top of the unit , not sure has yet what the load of that is .

spike47 · 13 Feb 2021

spike47 said:
There is also quite a large fan on top of the unit , not sure has yet what the load of that is .

I think also you are not supposed to be inside for more 3 mins at a time, it is on a timer , you have to put tokens in to start it up ! .

JohnW2 · 13 Feb 2021

spike47 said:
Going to use 6mm2

Depending on what sort of earthing system your installation has, it might be impossible with any length of any size cable ...

... with a 45A Type C MCB/RCBO, the maximum permissible loop impedance would be around 0.49Ω, and that is the total of the impedance outside of your house (from your house to the substation), over which you have no control, and of the cable within your house. Given that the former component (over which you have no control) might be as high as 0.8Ω, it obviously might be impossible to get the total down to 0.49Ω !

I think you need an electrician to look into this, and to make some measurements!

Kind Regards, John

spike47 · 13 Feb 2021

JohnW2 said:
Depending on what sort of earthing system your installation has, it might be impossible with any length of any size cable ...

... with a 45A Type C MCB/RCBO, the maximum permissible loop impedance would be around 0.49Ω, and that is the total of the impedance outside of your house (from your house to the substation), over which you have no control, and of the cable within your house. Given that the former component (over which you have no control) might be as high as 0.8Ω, it obviously might be impossible to get the total down to 0.49Ω !

I think you need an electrician to look into this, and to make some measurements!

Kind Regards, John

Have decided it is not going to be within the house cicuit ( Blue phase) , am taking a feed from one of the phases ( Yellow phase ) in another part of the farm building, and putting it in a room within the farm building, where we have set up has a training/weight lifting etc room , the 3 phase feed to the farm is a TT system, there is several earth rods around the buildings .

Spike

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RCD for professional sunbed circuit installation (edited.)

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