RCD protected Cables under Computer Flooring?

[Krispy]

Hi All

I’m after some advice, I am doing a restaurant fit out involving a number tables that have hot pots built into the table, for customers to heat/cook their food in the hot broths or stews what have you. These are 3kw and just fed from a fuse spur.

The cable run is LSF T&E on tray laid under the computer flooring, in which there’s about 60-70mm space from the cable to finished floor level. The computer flooring is wooden tiles with a layer of sheet metal around it.

My question is, do these circuits need to be RCD protected? My worry is that as the flooring has metal components, then they may have to be RCD irrelevant of the depth (if it’s like running in walls with metal parts). But as it’s flooring, and very unlikely to have any fixings put deep enough to touch cables, is it classed differently and could the go on MCBs?

 

[westward10]

Well the cables would have mechanical protection where they pass through the metallic parts and if no socket outlets are involved then you could probably escape an rcd but surely it would make sense to fit them.

 

[Krispy]

Well the cables would have mechanical protection where they pass through the metallic parts and if no socket outlets are involved then you could probably escape an rcd but surely it would make sense to fit them.

Well the cables would have mechanical protection where they pass through the metallic parts and if no socket outlets are involved then you could probably escape an rcd but surely it would make sense to fit them.

Thank you, yeah I will more than likely use RCBO’s but as there are 29 hot pots, I was just seeing if it was viable option as I have not seen any spec for these yet, they are imported from China so just in case they didn’t like RCBO’s and cause nuisance tripping, and not to mention it would save a fair bit of money having MCBS.

 

[davesparks]

Thank you, yeah I will more than likely use RCBO’s but as there are 29 hot pots, I was just seeing if it was viable option as I have not seen any spec for these yet, they are imported from China so just in case they didn’t like RCBO’s and cause nuisance tripping, and not to mention it would save a fair bit of money having MCBS.

If they 'don't like RCBO' s' and cause tripping then they are hardly likely to be safe to use are they!

 

[westward10]

Thank you, yeah I will more than likely use RCBO’s but as there are 29 hot pots, I was just seeing if it was viable option as I have not seen any spec for these yet, they are imported from China so just in case they didn’t like RCBO’s and cause nuisance tripping, and not to mention it would save a fair bit of money having MCBS.

Is that going to be 29 circuits. How are they to be connected.

 

[Krispy]

Is that going to be 29 circuits. How are they to be connected.

No not 29, so I’m going to do 15 rings and have two hot pots on a ring (with one odd one), as they’ve situated the tables in pairs. That’ll cut down the circuit numbers, and if for any reason they need go on their own circuit then the ring can be split back to two radials. They have got a sample table coming to site so I can take a look, but it’ll probably just be a fuse spur jigged into each table, wired through the void of the table.

 

[westward10]

What power does each one take.

 

[Krispy]

What power does each one take.

I’ve been told by my boss they are 3kW. On the drawing it’s shows a single socket in the table for it. That, and a Chinese YouTube video is all Ive seen so nothing on the spec yet so that’s all I can go with atm So I’ve just allowed a 32A Ring main for 2No, with nothing else on.

 

[westward10]

I would be looking at individual radials for a 3kw appliance.

 

[Krispy]

I would be looking at individual radials for a 3kw appliance.

Is that based on your experience, or is there something in the regs that recommends this? I actually agree with you on this, it would be better but when I was designing this, the thought of having 29 circuits just for the hot pots was I felt was a bit overkill. I’d imagine they just work as a tea boiler type load and flick on and off automatically as and when it needs to heat.

 

[westward10]

There is no Regulation as such but Appendix 15 suggests cooking appliances, not those exact words, above 2kw should be on their own circuit, see Regulation 433.1.204 but if they and the legs are evenly spaced it shouldn't be an issue. The biggest issue to me is 3kw loads on fuse connection units, they will be failing on a regular occurrence.

 

[davesparks]

Is that based on your experience, or is there something in the regs that recommends this? I actually agree with you on this, it would be better but when I was designing this, the thought of having 29 circuits just for the hot pots was I felt was a bit overkill. I’d imagine they just work as a tea boiler type load and flick on and off automatically as and when it needs to heat.

A circuit serving 2 points isn't much of a ring is it?

I don't see any advantage of this over installing a radial circuit, what was your thinking behind this?

 

[Krispy]

There is no Regulation as such but Appendix 15 suggests cooking appliances, not those exact words, above 2kw should be on their own circuit, see Regulation 433.1.204 but if they and the legs are evenly spaced it shouldn't be an issue. The biggest issue to me is 3kw loads on fuse connection units, they will be failing on a regular occurrence.

Thanks for your advice! I just looked, and as you rightly said, it does say about having cookers/ovens/jobs on their own circuits. Even putting them on 16A radials, I don’t know if I’ll be able to avoid putting them on fuse spurs as I don’t know how this equipment will come, if it’s pre wired with a small flex, but I’ll cross that bridge when I come to it.

 

[Krispy]

A circuit serving 2 points isn't much of a ring is it?

I don't see any advantage of this over installing a radial circuit, what was your thinking behind this?

The point was, a 2.5mm radial is ran to each hot pot, 16A radial for each. But as the tables are in pairs, with a 3m bit of 2.5mm I can link the spurs to make a ring, thous halving the amount of RCBO’s cost and saving capacity (14 single ways) on a board.

 

[davesparks]

Thanks for your advice! I just looked, and as you rightly said, it does say about having cookers/ovens/jobs on their own circuits. Even putting them on 16A radials, I don’t know if I’ll be able to avoid putting them on fuse spurs as I don’t know how this equipment will come, if it’s pre wired with a small flex, but I’ll cross that bridge when I come to it.

Of course you can avoid putting them on fused connection units, pre-wired flex or not.

16A radial with a 20A DP switch incorporating a flex outlet.

 

[James]

lets face it, if it was drawing 3kw continuously, the dam thing would be glowing in the middle of the table.

i would go with the fused spur at the table, gives a nice easy way to isolate it for cleaning etc.

i doubt it averages more than 1kw when it has reached its operating temperature.

from what i have read, i would probably be using 16A mcb or rcbo per table with an fcu for local isolation or maybe even a rotary isolator.

edit:
just noticed i have simultaneously posted almost the exat same as @davesparks
IT MUST BE RIGHT THEN!!!!

 

[davesparks]

The point was, a 2.5mm radial is ran to each hot pot, 16A radial for each. But as the tables are in pairs, with a 3m bit of 2.5mm I can link the spurs to make a ring, thous halving the amount of RCBO’s cost and saving capacity (14 single ways) on a board.

So the loads won't be evenly distributed around this 'ring'?

This just sounds like a bad, and arguably non-compliant, design to me.

2 appliances per circuit does halve the number of RCBO's yes, but it also double the number of tables out of action if a fault occurs and so doubles the losses the business makes if there is a fault.

 

[James]

So the loads won't be evenly distributed around this 'ring'?

This just sounds like a bad, and arguably non-compliant, design to me.

2 appliances per circuit does halve the number of RCBO's yes, but it also double the number of tables out of action if a fault occurs and so doubles the losses the business makes if there is a fault.

Agreed, £15 per table is not a lot in the scheme of things when if a table is out of action on a busy night you may loose £500 pet table per night.

 

[Krispy]

Agreed, £15 per table is not a lot in the scheme of things when if a table is out of action on a busy night you may loose £500 pet table per night.

So the loads won't be evenly distributed around this 'ring'?

This just sounds like a bad, and arguably non-compliant, design to me.

2 appliances per circuit does halve the number of RCBO's yes, but it also double the number of tables out of action if a fault occurs and so doubles the losses the business makes if there is a fault.

 

[Lucien Nunes]

IMO:
Plug table in under floor. Things get damaged and need to be replaced at a moment's notice. If service staff can't unplug a table they'll be swearing at you in various languages while rewiring your fused spurs with a penknife.

RCD protection and accessible isolation per table more or less essential, even without sockets. Drinks get spilled. Soup gets spilled and needs to be sponged. One day anything electric built into the table will get completely soaked.
Staff need to be able to isolate the table immediately without disturbing other customers.

Likely average load nearer 1kW per table. 3kW would see it boiling vigorously like a kettle. Typical electric hotpots are 1-2kW connected load but element rides on a thermostat. Sheathed heating elements can cause leakage but most likely in the portable appliance kind of range, unlikely to be over 1mA.

The scheme interests me because 40 years ago my family used to run the only all-fondue / table-cooked restaurant in the UK. None of these built-in electric table gubbins though. Metal (for oil) and ceramic (for cheese) fondue pots and mobile flambé station had spirit burners, fire kettles / hot pots burned charcoal, possibly the only electric devices were raclette grills. Table-cooking can bring a unique energy and sense of occasion to a restaurant. If one or two of our regular bon-vivant customers had parties in they would energise the whole place. We had years of fun but by gosh was it hard work.