earth testing a metal socket

[ElyDave]

REplacing the overheated socket in my son's room this afternoon, luckily only the front bit, the new one was a direct swap.  The metal box itself was connected to earth as was the faceplate, but it started me thinking, how do I test the earth itself?

I ended up using one prong od the multimeter to open the earth cover in the socket itself, and the other probe on one of the screws holding the faceplate on.  Indicated zero ohms (rather than open circuit), so happy with that, logic being that the screws are in continuity with both parts of the socket.

Anyone think of a better way?

[Kim]

The correct answer involves a long lead and a Megger, though a Socket & See type tester is a good start.

I would expect the socket to connect the earth pins and the faceplate screw holes internally.  Theoretically, this should be enough to earth the backbox via the screws, though I expect the regs require a separate lead.  You haven't tested that any of this is actually connected to earth, thobut.  Look for continuity to the earth pin of another socket, convenient bit of plumbing, etc.

What caused it to overheat?  Loose terminal screws or a wall-wart meltdown or something?

ETA: Oh, it was presumably victim to that ex-plug you posted yesterday.

[ElyDave]

correct on cause, it was that same fan heater, plug melted into socket.  Yes it does have an earth lead from the box to the faceplate, and all screws are bastard tight

[james]

Hi, I have a Martindale EZ150 socket tester that includes proper earth loop impedence testing if you'd like to try it, I've sent you a PM.

[Brucey]

Fuses do not always blow - fan heater plug, 13A fuse, Neutral completely melted, and of course plastic body so no earth

as Kim suggested, looks like a simple loose screw in the plug. Not a bad idea to open every plug (in high amp, oft-used devices especially) every year or so and check the screws are still tight. I have also seen the spring clips that hold fuses overheat and fail, presumably because they were just a bit dirty.

  You are right to change the socket though.  FWIW I have seen several socket switches fail by overheating when they have been repeatedly used to switch heaters, kettles etc on and off. Also the springy bits that ensure good contact with the prongs on the plug  in some sockets are noticably springier than others.   Probably worth spending the extra on a crabtree or MK socket...?

No reason to doubt the earth connection to the socket, and simple tests using a DMM are encouraging,  but testing it and being 100% sure of it requires some special test equipment.

 OTQ, how do folk feel about screw terminals bearing down on tinned cable ends? 

cheers

[grams]

A continuity test between the earth pin and a nearby metal pipe (water, gas or heating) is a reasonable approximation for checking the earth is connected.

I’d suspect that neutral wire was making a poor connection and got hot.

[Kim]

OTQ, how do folk feel about screw terminals bearing down on tinned cable ends? 

I reserve the right to do it when persuading fragile skinny cables that aren't carrying any current to remain unfrayed for the time it takes to do up the screw (usually low-voltage stuff like telecoms), but consider it a Mk 1 Bad Idea on mains wiring.  On the other hand, it probably made sense for appliance manufacturers to do when they were sold without a plug, as muggles doing a shoddy job at stripping/twisting wire is probably a greater risk than plastic deformation of solder over time.

The correct solution would be terminals with a clamping plate, and/or lugs or bootlace ferrules on the cable.  Not sure you can do either inside a BS1363 plugtop?

Loosely related PSA: Anyone still bodging up connections with choc blocks should get a bag of Wago connectors to play with.

[Polar Bear]

Wago is the way to go.

[chrisbainbridge]

Wago where have you been all my life! 

Moving into new house and replacing burnt bayonet sockets dangling from ceiling Rose with spotlights. All the loop wires replaced with wagos then changed the connectors supplied to wagos as well.
 Brilliant

[rr]

Variety box added to next Screwfix order.

Sent from my moto x4 using Tapatalk

[Brucey]

OTQ, how do folk feel about screw terminals bearing down on tinned cable ends? 

I reserve the right to do it when persuading fragile skinny cables that aren't carrying any current to remain unfrayed for the time it takes to do up the screw (usually low-voltage stuff like telecoms), but consider it a Mk 1 Bad Idea on mains wiring.  On the other hand, it probably made sense for appliance manufacturers to do when they were sold without a plug, as muggles doing a shoddy job at stripping/twisting wire is probably a greater risk than plastic deformation of solder over time.

The correct solution would be terminals with a clamping plate, and/or lugs or bootlace ferrules on the cable.  Not sure you can do either inside a BS1363 plugtop?

just for fun I have used bootlace ferrules inside a 13A plug but it was a bit fiddly and it felt like overkill.  Given a free hand I still prefer the old MK 13A plugs with proper post terminals inside them. At one time you could get those with a switch in too; all very sensible.

One of my former colleagues reckoned that the tin in the solder would be subject to stress/temperature induced twinning and that was the thing that allowed the screws to lose their grip over time.  Against that  screws could (even without a plate or a ferrule) probably be tightened more, and have a larger contact area in the joint to begin with. Swings and roundabouts I suppose.   When appliances were still shipped without plugs, often the wires would be bared for about 3/8" length with just the tips tinned. I always suppose that the idea was that the bared wire was doubled over before the screw was tightened onto the bare copper.

However I don't ever remember seeing a set of wires which were cut to length for safe 13A plug wiring. I consider it a failing of the design that if it is wired in the usual (casual) fashion, i.e. with nearly equal length wires, then the last one to pull out of the plug if the wire sees a hard tug  is the live wire.  Potentially this leaves the appliance unprotected and if the neutral wire is both disconnected and exposed by being pulled  out of the plug, live parts are then exposed. 

I have often wondered why they didn't route the live wire the long way round the middle of the plug, so that it would be first to get pulled out, rather than the last. Possibly there is a very good reason I have not thought of.

cheers

[Kim]

However I don't ever remember seeing a set of wires which were cut to length for safe 13A plug wiring. I consider it a failing of the design that if it is wired in the usual (casual) fashion, i.e. with nearly equal length wires, then the last one to pull out of the plug if the wire sees a hard tug  is the live wire.  Potentially this leaves the appliance unprotected and if the neutral wire is both disconnected and exposed by being pulled  out of the plug, live parts are then exposed. 

I have often wondered why they didn't route the live wire the long way round the middle of the plug, so that it would be first to get pulled out, rather than the last. Possibly there is a very good reason I have not thought of.

Presumably the idea is that the live has the shortest straightest path, so it can pull out easily.  Makes good engineering sense, but assumes the wires are cut and stripped to the correct lengths.  (The last new rewirable plug I encountered came with a piece of cardboard telling you the lengths in millimetres, but like most people with a box of reclaimed plugs, I don't see a lot of new ones.)

I suppose the rule of thumb applies that if you try to make something foolproof, you'll just get a better fool.

[ElyDave]

Wago is the way to go.

They do look like a good idea, I think I have maybe one or two chocolate block connectors left

[Kim]

Wago where have you been all my life!

Simlarly, the Chinese knock-off people have come up with these nice in-line connectors that Wago haven't invented yet: https://www.ebay.co.uk/itm/254389428444

I'm not sure I'd trust them for permanent mains installation (how flammable *is* that plastic?), but they're lovely for temporary lash-ups and low-voltage stuff.

[Brucey]

my vote for 'most sensible 13A plug design' goes to this MK (or the internally similar 'tough plug' version)



Cutting wires to length is made easier with this design and with double-insulated appliances the wires can be equal lengths.

 I had my misgivings about the cable grip design when I first saw it but in practice it seems to be quite reliable.

cheers

[chrisbainbridge]

When i was taught as a wee nipper by my father to wire a plug, all 3 wires were of different lengths and the insulation had to butt up against the plug screw terminal.

Insulation was cut with a knife which almost but not quite penetrated the rubber, so that the rubber could be removed and the wire was to be untouched.

multi-core wire was to be twisted clockwise to make it hold together as the screw clamped down. 

This was then reinforced by my teachers at school.  Woe betide me if a plug was not done properly! 

However he had spent National Service teaching Radar to servicemen which included repairing the equipment.

[Kim]

I had my misgivings about the cable grip design when I first saw it but in practice it seems to be quite reliable.

I really like those.  Much less faffing about.

The exact opposite of the ruggedised plugs with a cover where you have to pass the cable through the hole before wiring, which you will inevitably forget no matter how many you do.  (I once spent a couple of evenings making up BS546^[1] extension cables between cues, where the plugs and trailing sockets were both of that design.)


_{[1] The old round-pin plugs, which are alive and well (made from modern materials) for stage lighting, as there's a safety benefit to not having fuses in places you need a ladder to get to.}