Supply options for hot food counter installation

hertzal123 said:

fed from a built in 3 phase board

Not sure that I get this. If the board is built in, why not have one circuit per heater?

I don't have a problem with MCBs in series. From the sound of it, overload cannot occur, so they are for fault protection only. A fault should be a very rare occurrence and would it really matter if the upstream MCB had to be re-closed?

The big problem with those things is that they are on for a long time, get hot and may well all come on at once so the de-rating of adjacent MCBs should apply and too much diversity may not be wise

I'm assuming that each counter is sized for manning by a single person so is 16-20A each (that can be split between the phases or ganged onto one ) and if they are 3 phase or not the controls on each one them allow selection of  the upper/lower heat etc If they are the kind where the heat from below can be a water bath, then the elements can be the kind that trip RCDs so not too many on one final circuit.

Quite often you see them plugged in in via single or 3 phase BS4343 style sockets depending on the variant, to allow isolation and  pulling out to clean under or behind.

Mike

Chris Pearson said:

From the sound of it, overload cannot occur, so they are for fault protection only.

If the elements are concentric mineral insulated type, a fault can occur where the short to the element outer is at the neutral end, and this can create an overload condition on a fault to earth. A similar condition can happen for spiral-would air heating elements supported on insulation, if they are part of a Class I appliance.

I therefore always recommend providing overload protection for these types of heating element, or appliances containing them, especially if they are not pluggable type A appliances (i.e. come with a standard UK fused plug)

gkenyon said:

If the elements are concentric mineral insulated type, a fault can occur where the short to the element outer is at the neutral end, and this can create an overload condition on a fault to earth.

So the resistance of the fault is too high for the breaker to clear it?

Surely, an RCD would trip?

Been there, done that, a few days a go. My immersion heater seemed to work, but on transfering it to the new RCD-protected board, it promptly tripped. Live-earth resistance was about 700 Ω (and L-N an expected 19 Ω) so not enough to trouble the old BS 3871 Type 3 breaker.

gkenyon said:

If the elements are concentric mineral insulated type, a fault can occur where the short to the element outer is at the neutral end, and this can create an overload condition on a fault to earth.

So the resistance of the fault is too high for the breaker to clear it?

Surely, an RCD would trip?

Been there, done that, a few days a go. My immersion heater seemed to work, but on transfering it to the new RCD-protected board, it promptly tripped. Live-earth resistance was about 700 Ω (and L-N an expected 19 Ω) so not enough to trouble the old BS 3871 Type 3 breaker.

Chris Pearson said:

Surely, an RCD would trip?

Yes, but there isn't always a requirement for an RCD in TN systems. Because of the 'cables concealed in a wall requirement', it means most domestic circuits are, but it's not always the case in commercial/industrial installations.

Chris Pearson said:

So the resistance of the fault is too high for the breaker to clear it?

Yes, that's the point. Sometimes, as you say, the resistance is relatively high ... and I note that if the resistance really always were 700 ohms even when wet, that's potentially not an overload ... although other times, it's not as high, so you get a fault to earth that equates to a either a small or small to moderate overload. Often such faults are caused by pinhole leaks that form due to corrosion over time, so in an appliance where water is only present some of the time, fault-finding can be tricky, and sometimes the symptoms come and go.