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Emergency stop switch for study/lab (UK wiring question)

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splin:
I've wiring my new lab/study area but not yet plastered the walls.  It occurred to me that it would be a good idea to alter the wiring to include a master switch to disconnect all the sockets on the wall behind the bench to allow all the equipment to be turned off at night or when the house is unoccupied to minimize fire risk. Being in the UK these sockets are currently part of a 30A ring main (ring final circuit in modern nomenclature) using 2.5mm^2 twin and earth cable.

I'm thinking to use a 45A double pole cooker switch and rewiring these sockets as a spur off the switch connected into the ring circuit. The spur would be wired into the switch as a sub ring.  I realise it is not usually recommended to join any part of a ring circuit together anywhere other than at the consumer unit and it can cause confusion later. Worse, standard ring circuit tests would not pick up a break in the sub-ring created by the intermediate connection (think figure of 8 circuit created from the original single ring).

In this case however it should be obvious that these 6 sockets are spurred from the main ring as they can be isolated by the new switch, but there is the possibility of a disconnection/break in the new sub-ring. The break wouldn't be obvious but the 6.sockets would now be connected to a 30A breaker with under-rated 2.5mm cable. Is this acceptable? It wouldn't be any different to a break occurring in any ring circuit except that a standard house wiring check should detect that - but such testing happens very rarely, if ever in many/most UK houses until major rewiring is done. I don't want to have to rewire these 6 sockets with 4mm cable if at all possible.

Second question: should I use an emergency stop switch instead?  If so,  can you get them to fit a standard back box as all the ones I can find in a quick search seem to have their own enclosure or are panel mounted?

jbb:
I can't comment on UK wiring rules (especially the particularities of ring circuits), but here's my 2 cents:

* If it's not too hard, I suggest a separate ring circuit for the lab all the way back to your switchboard.  That way you won't take out your fridge, WiFi router etc. when you trip something.  How does the UK stand on Residual Current Devices (RCDs) (aka Ground Fault Current Interruptors GFCIs)?  They can be a bit prone to false tripping.
* If you want an isolator switch, use a switch.  If you want an emergency stop, use an emergency stop (you'll also need a relay).
* Make sure the lights will still work even if the switch is off

andy3055:
By courtesy of this site (https://www.flameport.com/electric/socket_outlet_circuits/ring_spurs_socket.cs4), you cannot wire more than 1 socket as a spur from the ring. So, you may want to run a completely new ring circuit for the room or the work bench sockets and have it on a separate breaker in the panel. If you need an emergency shutoff, make sure it is a 4 pole switch as the ring has to be broken from both ends. If not, it will still be alive from the unbroken branch of the ring. I say a 4 pole switch so that you can break the live and the neutral lines on both sides of the ring for total isolation.

If your aim is to simply guard against earth faults, as suggested by jbb, you can use GFCI protected sockets: http://internationalconfig.com/icc6.asp?item=72300-S-10MA

max_torque:
I have a lab bench in my electronics room that has "Build in" socket rails, and i simply wire these to a switch and plug them into the ring main, with a suitably fused plug. That way the bench is a "mobile" device.  The downside is that there is a total of 13A available to all the sockets on the bench (currently 10) but realistically, do you need more power (@3kW) than that?

It also allows me to include a plug-in isolation transfomer to isolate the entire bench if necessary

splin:

--- Quote from: andy3055 on December 01, 2019, 07:35:52 pm ---By courtesy of this site (https://www.flameport.com/electric/socket_outlet_circuits/ring_spurs_socket.cs4), you cannot wire more than 1 socket as a spur from the ring.
--- End quote ---

That used to be the rule but it isn't in the current regulations. You should not add a spur to an existing spur because spurs are usually wired with 2.5mm cable which is underrated for a 30A or 32A breaker - with one double socket spur it is considered unlikely that you would overload the 2.5mm cable. It could happen in fault conditions such as a short within the spurred socket (considered unlikely), but not due to faulty appliances as their mains plugs are fused.

However, I don't believe there is a problem if the spurs are wired with suitably rated cable. In my case I could use 4mm cable for the 6 sockets or connect them in a ring with 2.5mm cable, joined into the main ring via the isolating switch.


--- Quote ---So, you may want to run a completely new ring circuit for the room or the work bench sockets and have it on a separate breaker in the panel.
--- End quote ---

Unfortunately the regulations here mean that I can extend an existing ring but if I change any connections to the CU (consumer unit or breaker panel), such as putting in a separate ring or radial circuit to the study/lab,  I would have to pay to have it certified to our 'part P' regulations.


--- Quote ---If you need an emergency shutoff, make sure it is a 4 pole switch as the ring has to be broken from both ends. If not, it will still be alive from the unbroken branch of the ring. I say a 4 pole switch so that you can break the live and the neutral lines on both sides of the ring for total isolation.
--- End quote ---

A 4 pole switch wouldn't work as it would break the existing ring - the other sockets would still work but would be connected as two radial circuits using under-rated cable. My intention is to use a 2 pole switch connected to the main ring on side and to the 'sub ring' on the other. This is known as a bridged or figure 8 ring. AFAIK they are permissible and were even recommended in early documents. They are fairly common in UK housing, probably more by accident by design, when wiring is altered such as connecting two spurs but it isn't usually recommended as it can cause confusion later because the circuits are non-standard and can make it very difficult to work out the wiring connections. In my case though it should be very easy to work out as the six sockets are isolated by the switch.


--- Quote ---If your aim is to simply guard against earth faults, as suggested by jbb, you can use GFCI protected sockets: http://internationalconfig.com/icc6.asp?item=72300-S-10MA
--- End quote ---

The consumer unit already has a 30mA RCD (GFCI). It would be better to have a separate one for the lab but that would require rewiring into the CU and much more expense. Gotta love safety regulations which encourage 'bodgineering' - creative engineering solutions which meet the regulations but may be less than ideal or even less safe (but not in my proposal I hasten add!)

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