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Topic Title: Ring main crossover test
Topic Summary: Unstable contact resistance in sockets
Created On: 15 March 2019 12:36 pm
Status: Read Only
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 15 March 2019 12:36 pm
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I have been testing (my) ring mains prior to a CU move. Also took the opportunity to replace some older sockets. Thought some of you might be interested in the results.

On the cross connect test I got some quite unstable resistance readings on the new sockets (MK double switched) measuring using a plug in adaptor. Resistance would vary by nearly 1 ohm which is a multiple of the expected reading on these circuits. All the older sockets measured fine.

Connections were checked and were tight. Measurements on the socket terminals were fine suggesting a problem with the new sockets, or the plug in adaptor rather than the wiring, test meter, or test leads.

Further tests with a sample socket removed and the terminals shorted using a range of plug tops showed that the problem was with the sockets.

Contacted MK who after some communication said this was probably due to "a build up of the contact lubrication causing fluctuations and additional resistance this will decrease with use, as the lubricant is dissipated". If his is indeed the case I wonder why they lubricate at all if it has to wear off before a reliable connection can be made.

More likely I suspect this is due to the low test current of my MFT and oxides on the surface of the plug / sockets contacts. I don't have a reliable high current ohm meter so I cant easily repeat the tests to see if this explains the readings.

Has anyone else come across this sort of thing?

As far as I can see the OSG is worded to prefer front of socket tests (as opposed to terminal tests) and I cant see any reference to using higher current meters so if this is a general issue it might merit a mention in the future.
 15 March 2019 12:46 pm
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I usually find that pulling the test plug in/out several times stabilises the readings, this is usually on older sockets during testing after c/unit changes. I put it down to oxidation of the contacts. I have found similar on new sockets though when testing additions.
 15 March 2019 01:19 pm
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Depending on the design, some sockets have a greased spring loaded tip on the rocker that operates the switch contacts in the manner of pushing slightly to one side or the other of the pivot of a see-saw. The actual contact surfaces should be clean though. well, should be...
I'd be tempted to plug in a kettle or similar couple of kW load, as if the resistance remains 1 ohm, then the hundred odd watts that it would dissipate should be evident in short order.
On an older socket be aware that a few microns of copper oxide is an insulator, but the sort that will punch through and spot weld at a few tens of volts- so a test with a bit more 'sizzle' would probably pass.

regards Mike
 15 March 2019 02:44 pm
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I've found that some new double sockets read differently on either side by as much as 0.2 Ohms.whether or not this has anything to do with lubricated turminals i'll leave open. Generally, older accessories tend to be more reliable. Which seems to suggest that its likely to be modern suppliers and/or manufacturing standards.



 15 March 2019 04:36 pm
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Thanks all, yes different readings on different sides. I will put some load on them over the weekend and see how hot they get.

 15 March 2019 08:04 pm
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On both old and new sockets I occasionally get funny results, I found operating the switch a few times and inserting the plug a few times generally resolves it, if not it I test at the back of the socket and consider is it worth recommending replacing it if old/dirty/abused.
 16 March 2019 11:39 am
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I'd agree that seeing a few hundred mlilliOhms extra at a socket isn't uncommon - and also that removing & inserting the plug a few times usually brings things down to an acceptable level.

I don't recall noticing the same effect on lighting (which might be expected if it was the switching mechanism that was the cause) - although that might have been masked by the fact we don't usually have two lights next to each other we'd expect identical readings on.

- Andy.

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