Sunbed Supply

[APElectrical]

Upright Sunbed.
63A supply in 16mm SWA
63A type C MCB to deal with initial inrush of current.
100mA RCD was recommended by manufacturer As 30mA Would cause nuisance tripping.

Maximum earth fault loop impedance value for 63A type C MCB is 0.27.
Ze is 0.21

How do I meet this value? Is there anything I could change to install to increase the max EFLI?
Thanks

 

[Pete999]

Upright Sunbed.
63A supply in 16mm SWA
63A type C MCB to deal with initial inrush of current.
100mA RCD was recommended by manufacturer As 30mA Would cause nuisance tripping.

Maximum earth fault loop impedance value for 63A type C MCB is 0.27.
Ze is 0.21

How do I meet this value? Is there anything I could change to install to increase the max EFLI?
Thanks

What is the loading of the sunbed in KW? How did you calculate the cable size and the OCPD size?

 

[pc1966]

If your 100mA RCD is upstream of the cable you don't need to worry so much about Ze but you should still meet the disconnection time on a L-N short.

What is the supply PSSC and cable length?
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What is the surge like?
BS88 80A fuse is 0.44 ohm, so you could put that after the MCB and before the cable so a short is cleared in under 5s if your R1+R2 is below 0.23 ohm.
[automerge]1589811525[/automerge]
Ugly for sure, knowing the MCB and fuse have overlapping I-t curves, but it might be a way round the problem without looking at some fancy adjustable MCCB that would allow you to balance just the right fault-clearing time and (hopefully) just out of the switch-on surge trip region.

 

[APElectrical]

What is the loading of the sunbed in KW? How did you calculate the cable size and the OCPD size?

Sorry, have not installed the sunbed yet. Just quoting.
The only specifications I have are from the client, direct from the manufacturer. That included needing a 63A supply, type C MCB and no 30mA RCD . The KW rating was not on the email. I have carried out work at the premises beforehand so have got Ze value of 0.21
[automerge]1589812337[/automerge]

If your 100mA RCD is upstream of the cable you don't need to worry so much about Ze but you should still meet the disconnection time on a L-N short.

What is the supply PSSC and cable length?
[automerge]1589811055[/automerge]
What is the surge like?
BS88 80A fuse is 0.44 ohm, so you could put that after the MCB and before the cable so a short is cleared in under 5s if your R1+R2 is below 0.23 ohm.
[automerge]1589811525[/automerge]
Ugly for sure, knowing the MCB and fuse have overlapping I-t curves, but it might be a way round the problem without looking at some fancy adjustable MCCB that would allow you to balance just the right fault-clearing time and (hopefully) just out of the switch-on surge trip region.

The length of run on the cable is 10 meters above a suspended ceiling .
I haven’t installed it yet, just quoting for supply and came across this issue. Your idea of having the fuse after the MCB may be the answer??

 

[pc1966]

Looking at the Prysmian data sheet 16mm has 1.15mOhm/m for conductor and 3.7mOhm/m for the SWA, so R1+R2 about 4.9 mOhm/m and even for your initial case of Ze 0.21 and required Zs of 0.27 your limit is 0.06 Ohms so max length of 12m so just usable!

Using 3 core SWA so you have a 16mm Cu CPC also joined in parallel with the SWA at both ends gives an R1+R2 impedance of less than 2.3m Ohm/m so max length would be 26m which is a bit more reasonable margin.

So probably it is just OK as it stands, especially if you use 3-core.
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Looking at the book again, there is no disconnect time requirement on an overload (just protection against fire/damage obviously) so simply having a 100mA or 300mA instant RCD before the SWA cable would also allow it to be met easily.

 

[APElectrical]

Looking at the Prysmian data sheet 16mm has 1.15mOhm/m for conductor and 3.7mOhm/m for the SWA, so R1+R2 about 4.9 mOhm/m and even for your initial case of Ze 0.21 and required Zs of 0.27 your limit is 0.06 Ohms so max length of 12m so just usable!

Using 3 core SWA so you have a 16mm Cu CPC also joined in parallel with the SWA at both ends gives an R1+R2 impedance of less than 2.3m Ohm/m so max length would be 26m which is a bit more reasonable margin.

So probably it is just OK as it stands, especially if you use 3-core.
[automerge]1589813615[/automerge]
Looking at the book again, there is no disconnect time requirement on an overload (just protection against fire/damage obviously) so simply having a 100mA or 300mA instant RCD before the SWA cable would also allow it to be met easily.

Perfect. Thanks for your time mate.

 

[pc1966]

The figures I had might be DC resistance, so you ought to allow a bit of margin for AC, but hopefully you have some solution(s) to offer the client. Good luck!

 

[radiohead]

When I do these the manufacturers always ask for a 100ma RCD. I take this to mean there is considerable normal leakage to earth and run a separate 10mm copper conductor in addition to the copper cpc to each bed.

 

[pc1966]

When I do these the manufacturers always ask for a 100ma RCD. I take this to mean there is considerable normal leakage to earth and run a separate 10mm copper conductor in addition to the copper cpc to each bed.

Yes, if nuisance tripping 30mA ones then must be above 10mA. Though if fed with 3-core SWA you have two CPCs already (assuming armour properly terminated in it).

I have never seen one that close so wondering if they have cable gland holes, etc, you could use a SWA gland on?

 

[APElectrical]

Yes, if nuisance tripping 30mA ones then must be above 10mA. Though if fed with 3-core SWA you have two CPCs already (assuming armour properly terminated in it).

I have never seen one that close so wondering if they have cable gland holes, etc, you could use a SWA gland on?

I was planning on putting up a 63A vertical interlocked switched socket as there has to be a point of isolation locally outside the bed in case someone gets into trouble and sets the alarm off via a button inside the bed.

 

[bigspark17]

I have seen one of these in a local shop, it was on 30mA rcd and B63A mcb and had no tripping issues. 63A roatary adjacent, 16mm t&e cable.

 

[radiohead]

I have seen one of these in a local shop, it was on 30mA rcd and B63A mcb and had no tripping issues. 63A roatary adjacent, 16mm t&e cable.

That is not what the supplier specifies though.