'high security' ring main

With a possible move in mind, missus & I went viewing.

In one house I noted several red double 13A sockets in red. Previously I've only seen those in hospitals. Owner said that his son is an electrician and that these were sockets providing high supply security for computer use.

Presumably the purpose is to prevent or reduce nuisance tripping.

Unfortunately I didn't get a squint at the CU, but can anyone make an informed guess as to how the circuits are arranged please?

Just guessing - might he have installed class C MCBs to reduce the chance of tripping??

TIA

Reply to
jim
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I can't see any point in putting computers on a class C MCB unless you have a LOT of them and they are all starting up at once...

More likely they are on a separate circuit which is not RCD-protected, and the red serves to indicate that you shouldn't plug your lawnmower in. There may also be a UPS somewhere.

Reply to
Ben Blaukopf

It might be ring connected to a UPS under the stairs, or it might just have a big surge suppressor.

Reply to
Martin Bonner

Two options here:

First one is I would guess that he has wired a circuit with "high integrity" earthing. This is where you make sure that the earth connection is in a ring configuration (even on a radial circuit), and each end connects to independent terminals on each socket or accessory. This is for use with circuits that may normally experience high earth leakage currents. (Computer kit switched mode PSUs are often a common cause of high earth leakages)

The other option is that he has wired in a UPS and provided some uninterruptable sockets. The colour change would be significant here, since these sockets are unlikely to be rated for a full 13A load, and also will not disconnect in the event of an earth fault.

Reply to
John Rumm

Are these still commonly (or ever) fitted with non-standard sockets to admit only plugs with the "Earth" pin rotated through 90 degrees? A couple of buildings in which I worked had them for IT kit (with "clean" earths); and it struck me those non-standard sockets if still readily available wd be a better way to stop someone plugging the hedge trimmer into a socket which is not RCD-protected.

Reply to
Robin

I don't believe there is a requirement to identify or restrict sockets in this way (although it may be desirable from a users point of view)

Generally speaking the way to ensure the hedge trimmer is safe is to make sure all sockets that are likely to be used to feed it are RCD protected.

If for any reason there is one that is not, then it ought to be at least labelled as a minimum, but "unusual" sockets for the intended appliance are also a good solution here.

Reply to
John Rumm

Another possibility is that there are two types of mains, "clean" mains and "dirty" mains.

As I understand it, the clean mains sockets are on their own circuit with its own MCB at the CU, and only IT related gear and UPS'es are plugged into this circuit. The dirty mains are your normal appliances with motors which by themselves chuck out a lot of RFI, EMI , surges, brownouts etc. The objective here is to stop the other appliances's own noise from affecting the IT gear.

Reply to
Stephen

I vaguely remember a recommendation for labelling. When I installed my kiln sockets, following the regs as best I could, I did label them as not having the same RCD protection as the rest of my workshop.

I would expect this ring in the house is simply a separate ring and MCB, maybe a split-load CU, to avoid nuisance trips from other rings.

Reply to
Andy Dingley

Most probably high integrity earthing. See

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Reply to
Peter Parry

I'm still wondering why in the UK (EU?) RCDs trip at around 30 milliamps, but in Canada and the U.S. their GFIs trip at 5 milliamps.

It's the current level that kills; even 30,000 volts of static won't hurt you, though it may make you jump, so saying that N. America uses half the voltage we do for domestic supplies is beside the point.

Reply to
Windmill

I have not seen a RCD in a hospital (for ward use) on any work I have done. The nice outside water feature had one.

Reply to
ARWadsworth

'Walsall gauge', as it was known, had all 3 pins rotated by 90 deg. MK have their own non-standard version of the BS 1363 plug & socket, which uses a T-shaped (in cross-section) earth pin, like this

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Reply to
Andy Wade

Medical equipment usually has really expensive ultra low leakage isolated power supplies.

I had one once, and the mains side was even all waterproofed, presumably so if a drip bag springs a leak and pours salt water all over the ECG machine, there's still no way it can become live.

Reply to
Andrew Gabriel

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But I can't find a picture of the ones I recall with just the earth pin rotated so my memory may have been at fault; apologies for that.

Reply to
Robin

I can not see why anyone would do this in a domestic house unless they had a very large number of computers. A couple of laptops or a couple of PCs do not necessitate high integrity earthing. An UPS supplying outlets is interesting, but generally someone uses a local UPS re USB/ Serial control of shutdown.

Computers leak a small amount to earth by design. A circuit should be designed so this leakage does not pre-sensitise any RCD protection, that is to say limit the number of computers per circuit by separate RCD or individual RCBO. A 30mA trip generally trips at 22-23-25mA so you do not want to go even half that with the PCs (think most are 1.5mA, limit being 3.5mA).

High Integrity Earthing is just that. For a ring the CPC are connected to separate earth terminals in each socket and back at the CU. For a radial another CPC is run from the last socket back to the CU, which is tedious, typically 4mm 6491X Gr/ Ye would be used as CPC re sized sufficiently to not need protection. Having twice as many screws to come loose on a 1.5mm conductor is interesting, but hey ho :-)

There was a myth locally, but it soon died out when the actual leakage current was considered and more importantly measured with a clamp meter. You would need to have a lot of PCs, if it were an issue on a radial circuit it would be cheaper to fit an RCBO to that circuit or circuits.

Hospitals use 10mA RCBO in many places, there are separate "zones" re electrical design as you go from ward to operating theatre... oxygen is never a nice gas to have around.

Reply to
js.b1

Ooo, any advice on that? My workshop tenant is a potter and as part of the rewire I'm planning on putting in a new seperate circuit for a kiln. He's currently got a little one plugged into a 13A socket (I think of it as a mini top-loader). My initial draft design notes have been to treat the new kiln as a type of high-power electric cooker. I'm slightly stymied in not knowing the typical power draw of an electric kiln, and suppliers' websites aren't much help.

JGH

Reply to
jgharston

Well so did the cleaner that retired last year.

Now when I rewired St Catherines (Doncaster) Hospital x-ray department 2 years ago there were no RCDs.

Nor were there any RCDs on any of the wards that I worked on where I added extra sockets.

Reply to
ARWadsworth

Separate radial, no RCD, round blue sockets, labelled as "No RCD protection". It also meant I had to go to RCBOs on the rest of the small shed CU.

This is a glass kiln, possibly a second one later. Ceramics are usually bigger, slower and more powerful than warm glass. It's 2.4kW, the bigger one on the future might be up to 5kW (I'd want more bench space before needing anything bigger).

Glass cycles are more complex than ceramic - they involve a ramp up to one temperature, short hold, a full power heating to working temperature, very short hold, crash cool to a lower annealing temperature and long hold, then a very slow cooling. A working cycle might take 3-4 hours, but the kiln is running at full power for only a fraction of this - 15-20 minutes, with about an hour at half-ish power.

Reply to
Andy Dingley

Didn't your fingers get all shrivelled up ?

Derek G.

Reply to
Derek G.

In the US, GFI's tend to protect just a single (or duplex) outlet, and sometimes a couple more daisy-chained from it, but typically just a few appliances max.

In the EU, an RCD tends to protect a whole circuit at a minimum which is much bigger, and thus many more appliances.

It's a function of the current, and the duration, and the path through you that kills. 30mA doesn't kill for the length of time it takes an RCD to trip. Changing to lower value tripping with multiple appliances generates more nuisance tripping, without saving any more lives (since no one has died at 30mA trip as far as I know).

I use a 10mA RCBO for my outside socket circuit, but although there's more than one socket, there's never been more than one appliance plugged in in total, either hedge trimmer or lawn mower. This is probably overkill ;-), but most of the hedge trimming involves standing on a tall stepladder, so I'd quite like to go better than not being electrocuted, and not even fall off the ladder. Having said that, I have in the past cut through the hedge trimmer flex when it was on a 30mA RCD (before I had the outside sockets), the current that tripped the RCD can only have gone through me, but I didn't feel it.

I have since replaced the hedgetrimmer cord with 3-core flex, so that if I cut through it again, a connection to earth is pretty much guaranteed, to ensure the RCD trips quickly. The earth conductor terminates without connection in the safety break connector, as the hedge trimmer itself is double insulated, as are most such outdoor appliances for other safety reasons.

Reply to
Andrew Gabriel

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