When my kids were growing up I had two consumer units in the workshop. One, which supplied the lights and a couple of 13A sockets in a ring main and the other for my machines. This had a 100A isolator, similar to the 32A one shown earlier, but was before the consumer unit. I could therefore isolate and lock out (padlock through the locking holes) all my machines from use by curious fingers with one ‘master switch’. As it turned out, none of my 3 boys had the slightest interest or intention of ever going into the workshop! I’m not sure any of them know which end of a hammer to hold!
Master switch for a circuit of sockets?
- Thread starter akirk
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guineafowl21
Established Member
No, a ring main uses undersized wires relative to the current rating of the circuit. Because the wires loop back to the consumer unit, they are effectively in parallel and you get away with it.
If the ring becomes broken, you then have two radials, each with undersized wires. This will be a silent fault as everything will continue working, but there is a chance of overloading the circuit now.
Any circuit will cause problems if not properly installed or develops a fault, of course, but it’s the silent nature of the broken ring main fault that many have a problem with.
NetBlindPaul
Established Member
A rotary isolator could be utilised, but to use all 4 poles they all have to be suitably rated and timed for correct switching if you are going to break each leg.
I prefer radials, rings are “ok” until they are not modified correctly, which is common for DIY works by people who do not understand the way a ring must be constructed to be safe.
A ring, when correctly designed and installed cannot be overloaded unless a socket is overloaded, the issue is, knowing what the loads are when the ring is to be designed.
A correctly designed and installed radial circuit cannot be overloaded either.
A radial is safer for incompetent DIY works because as long as a 2.5 live conductor is used and it is correctly designed and installed initially then it is difficult to modify to overload the cables. It is possible but this would generally involve insulation.
A red/yellow isolator operating mechanism is designated for emergency switching, a black/grey operating mechanism is designated for routine isolation, such as for mechanical maintenance, or end of day isolation.
Personally if my client insisted on a ring circuit, and a means of isolation in the supply, I would design and implement a “lollipop“ circuit, with a suitably rated cable to a sutably rated isolator as a radial and originate the ring circuit from the isolator.
One needs however, even if using RCBO’s mindful of the 10% maximum leakage on an individual earth leakage device as per BS7671.
If a full 32 is drawn from a broken ring, then even in the optimum scenario, the 2.5/1.5 flat win & cpc cable would be overloaded.
With higher earth leakages on equipment due to internal electronics, switched mode power supplies, LED lighting, variable speed drives, filters for EMC compliance etc. a supply circuit may well require a redundant cpc path.
A ring may well provide this, a radial will not.
Hence why singles (6491x or b, a6491aka H07V-R or H07V-U/H07Z-R) in pvc tube with metalclad sockets is a better option.
I prefer radials, rings are “ok” until they are not modified correctly, which is common for DIY works by people who do not understand the way a ring must be constructed to be safe.
A ring, when correctly designed and installed cannot be overloaded unless a socket is overloaded, the issue is, knowing what the loads are when the ring is to be designed.
A correctly designed and installed radial circuit cannot be overloaded either.
A radial is safer for incompetent DIY works because as long as a 2.5 live conductor is used and it is correctly designed and installed initially then it is difficult to modify to overload the cables. It is possible but this would generally involve insulation.
A red/yellow isolator operating mechanism is designated for emergency switching, a black/grey operating mechanism is designated for routine isolation, such as for mechanical maintenance, or end of day isolation.
Personally if my client insisted on a ring circuit, and a means of isolation in the supply, I would design and implement a “lollipop“ circuit, with a suitably rated cable to a sutably rated isolator as a radial and originate the ring circuit from the isolator.
One needs however, even if using RCBO’s mindful of the 10% maximum leakage on an individual earth leakage device as per BS7671.
If a full 32 is drawn from a broken ring, then even in the optimum scenario, the 2.5/1.5 flat win & cpc cable would be overloaded.
With higher earth leakages on equipment due to internal electronics, switched mode power supplies, LED lighting, variable speed drives, filters for EMC compliance etc. a supply circuit may well require a redundant cpc path.
A ring may well provide this, a radial will not.
Hence why singles (6491x or b, a6491aka H07V-R or H07V-U/H07Z-R) in pvc tube with metalclad sockets is a better option.
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Fascinating - thank you 
amazing what knowledge people have!
I get the logic behind singles as effectively in layman’s terms you are using higher quality wire- I assume the conduit is because singles don’t come packaged in a tough outer as ‘normal’ cable does? But why metal sockets?
amazing what knowledge people have!I get the logic behind singles as effectively in layman’s terms you are using higher quality wire- I assume the conduit is because singles don’t come packaged in a tough outer as ‘normal’ cable does? But why metal sockets?
So when I was refurbing my workshop (single garage) I got a mate who is a sparky to do the job for me. It's on it's 32A ring with an isolator for the complete ring. I have 10 might be 12 double sockets (never will all the sockets be powered/used at the same time). On one wall I have two doubles, for my large machinery Bandsaw, Mitre Saw, Thicknesser (when in-situ etc) and my dust extractor (rather than shop vac) the extractor is always powered on. These four sockets are behind a big red/yellow stop/kill switch (twist to activate push to stop - don't know proper name) I'd installed this previously so it was reused (but properly!). If the extractor is on my family know big tools are in use and I know the 'big' machines are powered.
Padster
Padster
guineafowl21
Established Member
Singles are more versatile in conduit, and also have a full-size earth. Standard T&E has a smaller earth wire. And yes, you’re not supposed to have single-insulated wire exposed.Fascinating - thank you
I get the logic behind singles as effectively in layman’s terms you are using higher quality wire- I assume the conduit is because singles don’t come packaged in a tough outer as ‘normal’ cable does? But why metal sockets?
Standard white plastic sockets are very brittle. Just the other day I tripped over a machine flex and broke the front out of the socket.
NetBlindPaul
Established Member
Metal sockets because the back boxes are designed to take conduit, and sockets are often mounted in pairs and you can connect the two with a metal coupler and female bushes which gives you a solid mechanical link.Fascinating - thank you
I get the logic behind singles as effectively in layman’s terms you are using higher quality wire- I assume the conduit is because singles don’t come packaged in a tough outer as ‘normal’ cable does? But why metal sockets?
Another thing I hate about ring mains is testing them, that can be a real pain especially if the board is tucked away under the stairs. It involves completely disconnecting both ends of the ring so you have six wires, the first test is to confirm the ring is intact. Connect a L to N and the other N to CPC and put meter between other L and CPC which must show continuarity. If not the fun now starts! The other test is to confirm that the ring has not been bridged by additional spurs or add ons. So here you link the L and N of one cable to the L and N of the other cable but cross them over, L to N. This effectively puts the live loop impedance in parallel with the N loop impedance and therefore the resistance measured between the L & N at any socket should be the same, you allow a 0.05 ohm difference. So now just measure this resistance at all sockets on the ring and again if any major differences then the ring has been bridged and again the fun starts.
Another advantage of singles is that you are not restricted to the CPC size in T&E, which is 1.5 mm for both 2.5 & 4.0 mm T&E. Not so important in smaller domestic installations but with much longer runs in an industrial setting you would run the same size CPC as the conductors.
Fully agree in the industrial enviroment but now there are back boxes that accept the Pvc conduit and fittings including couplers. You can tell the difference because the conduit type are not made of that brittle plastic and can be drilled using a hole saw. PVC Switch and Socket Boxes for Conduit
Another advantage of singles is that you are not restricted to the CPC size in T&E, which is 1.5 mm for both 2.5 & 4.0 mm T&E. Not so important in smaller domestic installations but with much longer runs in an industrial setting you would run the same size CPC as the conductors.
Yes thats a good solution, but why not try and steer your clients on the basis of I have more knowledge and experience than you so please take my advice.
guineafowl21
Established Member
If that’s the case, do you ever test them at a convenient socket?
TheTiddles
Established Member
Metal sockets is a good call, I’ve got a broken plastic back, can’t remember when or how I did it, the replacement is sat next to it, one day I might just do it...
I’d prefer a good DIY job to a bad “trade” job, I’ve seen live pins, exposed conductors, won’t mention what someone told me they saw on a submarine once
Aidan
I’d prefer a good DIY job to a bad “trade” job, I’ve seen live pins, exposed conductors, won’t mention what someone told me they saw on a submarine once
Aidan
You have to test all sockets, not only the convenient ones. Inspection and testing is only valid if done completely and meticulously using in cal instruments. Tiddles, I would say your broken backbox is the normal brittle domestic type, the one you get areas that can be knocked through, the ones for conduit are not brittle but made of a more pliable plastic that is eaily drilled. If any one is looking for suitable JB's for conduit then Wiska has a really good range:
https://www.tlc-direct.co.uk/Products/WK308B.html
and for your SWA's Wiska Earth Clamp Bar for SWA Glands for WK308
Once upon a time gas pipes were black iron, now yellow MDPE and compressed air ring mains in galvanised steel but now in polymer but I did use metal for most industrial installations although I also used a fair amount of Polycarb and GRP.
guineafowl21
Established Member
Yes, I meant doing the ring main tests - end-to-end and x-connect - at a convenient socket instead of a cramped consumer unit.
sometimewoodworker
Established Member
That can’t be done as you have to isolate the ring final that you are testing from all other circuits, so must be done at the CU wherever it is, I have sympathy for sparkles and the contortions they have to go through.
Your builder/sparky will probably give you some good options and will apply the rules to your property. If you give him your use spec, isolation from kids , load on circuits. Your home set up may dictate the options available/affordable. The workshop consumer unit may be run directly from the main house consumer unit in which case they will have all the above options available, However if its run off a spur from the nearest ring in the house, then your options may be limited or he will ask if you want to go to the expense of laying a new cable.
Sideways suggestion of using 4mm cable would provide future proofing. In my experience as the main user of the workshop, only one tool is used at once so there is little danger of overloading a 2.5mm cable. However one day you might get some 3phase beast of a saw or an electric arc or something needing more beef and 25m of 4mm cable will cost you about £10 more.
As was said earlier a double pole cooker switch would be cheaper than the industrial ones quoted above, but if it were me, I'd spend extra to get the electrics future proofed, especially if you are having it done professionally (as the law requires these days). Your suggestion of an isolation switch seems a good one if others (especially little others) could get access to you workspace.
History: Ring circuits were a British innovation designed to aid post war re-construction, they are resource efficient a they use less copper than equivalent radials, so planet friendly. They are pretty universal in UK and Irish homes and a few British influenced countries eg Singapore and parts of China! They do have advantages such as providing two routes to earth protection, but they are harder to test and if part of it fails they risk being overloaded - suppose you have a 3kw heater on in the winter with something else that is intensive. The sparky will know what is best once you told him your requirements.
I've recently discovered that its the law, or maybe just recommended, that domestic properties have an electrical inspection every 10 years. A few oddities were found in our house/garage, loose connections and a faulty outside appliance, so if you are having work done it may be worth having a full inspection, not cheap about £40 per hour and depending on the complexity of your property would take one or even two days. Oh and the (early 2000s plastic consumer unit now not good practice (route cause of fires) and needs to be replaced by metal - again - maybe we will go back to porcelain...
Good luck Tom
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sometimewoodworker
Established Member
There is a very significant expense involved in any modification to wiring unless you already have a metal CU and RCBOs (or one of the other acronyms) as I am just finding out as you are virtually guaranteed to have to install a new CU and they run at around £600 I‘ve just had inspections done to 2 properties and its over £1,000 each for the inspection CU and minor work needed.
Steve_Scott
Established Member
How? If the circuit protection is adequately sized for the conductor size and installation specifics, the cable can’t become overloaded?
I have no issues with ring finals but a break in the circuit creates an undersized and inadequately protected radial and even if the ring is complete, plugging appliances in close to the start/end of the ring can cause current imbalance, particularly in a long circuit. However... it’s a lot easier to pull 2.5mm cable!
Steve_Scott
Established Member
Plastic CUs are not the root cause of fires... poor terminations/resistive heating faults are the root cause... plastic CUs just happen to burn and spread the resulting fire.
But a switch is not an isolator and vica versa.
Yes plastic CUs do burn and spread the resulting fire, often not helped by being under the stairs and burried in allsorts of stuff someone has put there out of the way, similar to stacking a bonfire! The idea of metal units is to contain the thermal event, but regular test and inspection would probably be more effective, just like people who never test their RCD's on a regular basis.
