Differences in Zs between left and right sockets in a double outlet

[SW1970]

Hi, did a PIR on a house yesterday, early 90's build with lots of MK double sockets. Strange goings on with Zs - the left and right sockets were significantly different on many of the outlets, e.g. 0.4ohm left, 0.6ohm right. Any thoughts - dodgy batch? For a piece of brass or copper about 8cm long can't understand why readings are so different. Thanks, SW

 

[yellowvanman]

Contact resistance of switches!

 

[Spudnik]

Yup, resistance and use of the socket would create this situation.

One was probably used more that the other, which means the unused one may have oxidised slightly.

Push the plug of your tester in and out a few times of the suspect socket.

 

[Mark_Burgess]

As Jason says, only way you will get them similar also it to flick the switch repeatedly.

This is also common on cheap rubbish like Marbo.

 

[Superspark23]

flick the switches severell times, and pull the plug in and out a few times. Should get the readings more consistant.

 

[Mark_Burgess]

Not keen on volex then mark?

Nah, I am keen to try the Hager Sollysta but just cant get my hands on them.

 

[SW1970]

Ok, furthering this on a bit; adding some sockets into a ring so carried out testing around the ring to check for spurs, bridges etc. Cheap accessories from 1980s rewire. Readings are still significantly different even after lots of "plug thrusting" (that's my term for a plug giving it big time to a socket, you get my drift). For example:

Before plug thrusting:
Left: 1.4ohm Right: 0.72ohm

After plug thrusting:
Left: 0.97ohm Right: 0.69ohm

If I'm on the lookout for bridges on the ring, I'd expect <0.05ohm difference between different socket outlets on a L and N cross connect test, but with these sorts of magnitudes of differences caused (apparently) by oxidation, this test doesn't have much value.

In this scenario, should it be deemed that the accessory has failed and should be replaced to allow decent readings to be taken? Or any challenges on the 0.05ohm threshold? Thanks folks.

 

[Deleted member 26818]

Of course, you could always remove the socket front, and measure from the terminals.
I once was at a customer's house installing an extra socket-outlet in their hall, they hadn't long moved in.
The lady of the house unformed me that one outlet of one of her kitchen double sockets didn't work, and asked me if I could change it.
I just removed the plug from the other outlet, plugged it into the defective one a few times, untill it worked.
Probablly not the best course of action business wise.

 

[somersetsparks]

In this scenario, should it be deemed that the accessory has failed and should be replaced to allow decent readings to be taken? Or any challenges on the 0.05ohm threshold? Thanks folks.

I would only fail in this instance if the difference pushed it over the max zs.
If I do a ring test on a house with this problem I usually use my robes to do it as you can scratch the brass inside socket and get a better reading then if you just pushed a plug in.

 

[soulman]

i have also had this problem testing a ring a few years ago as spinlondon said unfortunately you have to remove the socket and test from the terminals. This was on the metallic type sockets and they were brand new, thought my tester was broken.

 

[WDMDL]

I've got the BS1363 test lead for my MFT, a proper socket test adapter, plus the old "dodgy plug".

The "dodgy plug" has had its pins purposely roughed up to bite through the tarnish on socket contacts.
It's not exactly GS38 but avoids pulling socket fronts off, balance of risk etc

Also the 3kW fan heater comes in handy to clean the switch contacts up too... a few make/breaks under full load generally sorts them out.

Simon.