ring final circuit

[thomo27]

Evening all Im looking to wire a ring main through a contactor with a stop start mushroom button that can be locked off with a key..

Im unable to download paint.net so ive created a drawing( a bad one) showing how I plan to wire this but would like to some advice/thoughts on how it could be improved..

basically its a 4mm ring from the DB into a contactor out of the contactor into each socket with the ring final circuit finishing back at the contactor..

The contactor is to be energised with the stop button unlatching the contactor and then a key to lock off then a start button to energise the contactor..

I obviously need to supply the stop start and then take a cable back to the contactor to energise the coil

cheers

Ste

Untitled

• thomo27
• 16 Aug 2013
• 1

 

[flameport]

Forget about a ring and just install one or more radial circuits from the contactor.

 

[EFLImpudence]

Not sure why you are using 4mm² for a ring or indeed a ring for two sockets.
A radial will be satisfactory with 4mm² and 32A MCB or 2.5mm² and 20A MCB.
So, with a radial you will just need one cable

Why do you need a contactor? Is it a low current switch?
What exactly are you trying to achieve?

 

[securespark]

Bit more info, please. Is there a reason it's a ring final? And a reason it's 4 milli?

 

[riveralt]

A Three Ring Question/Answer

 

[thomo27]

the spec for the job is a 4mm2 ring main feeding about 8 sockets at workstations (its a science lab) they want the teacher to be able to isolate the sockets with a stop button inc a key lock and then be able to re energise with the start button..

 

[securespark]

Just as an aside, wouldn't it make more sense to put the lock on the start?

 

[PrenticeBoyofDerry]

It would be possible to design a ring final circuit, that has lockable emergency stop buttons. Are the stop buttons required for every socket individually or banks of socket?

 

[thomo27]

Just as an aside, wouldn't it make more sense to put the lock on the start?

yes

 

[thomo27]

It would be possible to design a ring final circuit, that has lockable emergency stop buttons. Are the stop buttons required for every socket individually or banks of socket?

only 1 stop/start is required to isolate all sockets

 

[PrenticeBoyofDerry]

only 1 stop/start is required to isolate all sockets

Then in that case a radial socket would likely be a better choice of circuit.
Has it been specified to be a ring final circuit for any special reason?

 

[davelx]

only 1 stop/start is required to isolate all sockets

Then in that case a radial socket would likely be a better choice of circuit.
Has it been specified to be a ring final circuit for any special reason?

Probably because the people spec'ing the work don't have a clue and have only heard of "ring mains" 'cos they have them at home.

 

[OwainDIYer]

If you put each leg of the ring on a separate pole of the contactor, then the pole on the contactor that closes first/opens last will momentarily be taking the full current of the ring. This will lead to uneven wear.

 

[ericmark]

I would have been more inclined to use a moulded breaker with a solenoid release as used with RCD modules than a contactor.

However I would if feeding a contactor use just two contacts not 4 the forming of a ring would be after the contactor if required.

Size of area supplied will alter both size of cable used and if ring or radial think limit with 2.5mm ring is 106 meters of cable with 4mm more like 140 meters and as such it is rare to need 4mm although I did install a 6mm ring in a warehouse with radial drops to each socket for cleaners use but this was a very large building.

Any emergency stop system does need to comply and first stage is a risk assessment. Where permanent injury would result in any failure of the system then everything needs doubling up. It seems daft but to comply two contactors even if supplied from one emergency stop relay and the emergency stop would need two sets of contacts today often one normally made and one normally broken but this would depend on the relay used.

Since in a school the risk assessment would be a must and until you know what risk you are protecting against there can be no real answer. With high risk the moulded beaker approach would clearly be out as it does not fail safe. At least the ones I used did not.

Do insure you have good insurance in place before undertaking this work which of course also means you will need to be able to prove you have the skill required which in turn means passing some exams so there is a paper trail.

 

[JohnW2]

If you put each leg of the ring on a separate pole of the contactor, then the pole on the contactor that closes first/opens last will momentarily be taking the full current of the ring. This will lead to uneven wear.

Theoretically true, although I doubt that the contactor would often (if ever) open or close in the face of a high load, so it's probably not significant. Indeed, if this is intended only as an 'emergency' provision, it's possible that the contactor contacts would almost never be open.

Having said that, and as others have said, if I were asked to design such a circuit (and if I were 'qualified' so to do), I think I'd almost certainly go for a radial circuit, which would simplify things and also remove some possible issues such as that which you mention.

What confuses/surprises me a little is that if the specification of required work goes as far (and into such detail) as to demand the (rather surprising) 4mm² ring circuit, that it doesn't also specify in detail how the on/off switching of the circuit is to be achieved.

Kind Regards, John