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Raptor0014

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Morning all,

I was working at a property yesterday and came across something I've not seen before (which isn't unusual given I haven't long since qualified) and as I thought it could be potentially dangerous I thought I'd ask on here.

One of the jobs the client wanted was a new socket outlet in the bedroom for a TV. Which in itself wasn't a problem (apart from the battle to fish the cable). I first went into the attic to see if the existing upstairs ring was fed anywhere from there as the bedroom is directly above the garage where the consumer unit is. However only the lighting cables were up there.

So I removed a couple of socket covers to check which direction the cables where coming in and they all fed from below. Pulled the carpet back and it was chipboard and the client didn't want holes cutting in. I came up with a plan to fish a cable from one existing socket, up into the attic across to the other wall and down inside the partition as the client also didn't want chasing into the wall if at all possible.

I checked the CU and there were two socket MCBs. One labelled L and labelled K.

L = Lounge
K = Kitchen.

I plugged in my buzzing plug tester so I could hear which MCB would kill the sockets. The one it was currently in on the socket I intended to use was on K MCB. I couldn't get the fish rod up the wall from that socket so moved to a different one (on the same wall in the same room) and before I took the cover off, and because I am CDO, I checked the socket with my buzz tester and found the socket was live! This socket was on MCB L!

I had another battle getting the cable up in to the attic but eventually did it to create the single 2 gang spur for the TV & Sky Box. But I was just surprised to find two sockets in the same room on the same wall on different MCBs.

I know that in 16th Edition this was apparently a suggested method so that if one MCB trips there are still some sockets available. Or so I was told by an experienced spark.

But if you were doing a full ECR and found that what would you label it as? A C2 or C3 or just make a comment?

To summarise what I thought was a bit wrong (but happy to be educated otherwise):

  • No dedicated ring or MCB for the upstairs sockets
  • No labelling/schedule on or with the CU to explain how the sockets are wired
  • Sockets on same wall in same room wired to two different MCBs

Also, just for my education, I was surprised that when I checked the new spur I got a reading of 0.23 ohms but at the socket I took the spur from I got a reading of 0.29 ohms. My Megger is a 1741+, is new and was calibrated properly before it was sent to me. I would have thought being a spur the reading would be slightly higher and the only thing I could think was the existing socket now had 3 x 2.5 cables per terminal and the spur had 1 x 2.5 cable per terminal. I also replaced the existing socket cover as the plate inside the back was cracked. So both had new 2G covers.

Sorry for the long post. Wanted to make sure I'd given all the info.

Regards

Carl
 
Quite common in houses where there have been alterations to find sockets in the same room / wall on different circuits. I once found a bungalow bedroom with 3 different socket circuits in the (fairly small) room. Ideally the consumer unit / fuse box should be labelled to reflect this, but so often it is not. But you will always check any socket you work on is dead, and not assume because you've turned one of some others will be turned off!
 
as above. ^^^. and regarding the readings, could just be a bit of contact or switch resistance on the existing socket. try plugging tester in and out a couple of times and operate the switch/es a few times.
 
as above. ^^^. and regarding the readings, could just be a bit of contact or switch resistance on the existing socket. try plugging tester in and out a couple of times and operate the switch/es a few times.
I did that as thought that could be the case. Even rechecked the terminations were all tight. But still got the same readings. It was safe so wasn’t overly concerned. Just thought it odd.
 
Measurements at sockets [(R1+R2), and Zs] can vary a fair bit, due to things like poor contacts to the plug pins, poor switch contacts, loose terminals inside. Wiggling the test plug a bit can sometimes help overcome poor contacts. And switching on/off a few times might help the switch contacts (or make them worse sometimes). If I get wildly varying readings, I typically unscrew the faceplate and check for loose terminal screws.
 
Might be worth plugging in a 2kW heater and power cycling it using the socket's switch and re-testing.

As above, usually it is poor contacts but occasionally you just get a "bad" socket where no reasonable amount of inserting/removing a plug and cycling the switch makes a difference (even from the good brands).
 
I have found some brands of socket give consistent resistance readings and others have a much higher variance. This can be due to different alloys used in switch contacts etc; the most durable contacts do not always offer the lowest or most stable resistance. In this case the resistance in the ring point socket itself seems to have exceeded the additional resistance of the spur cable.

Might be worth plugging in a 2kW heater and power cycling it using the socket's switch and re-testing.

This usually 'fixes' high contact resistance. We do a similar thing with a 500W halogen lamp in theatre lighting (where the sockets aren't switched but the contacts of rarely-used sockets sometimes build up tarnish). When you have 24 sockets all in one place with exactly the same length of circuit, you get to see clearly how the contact resistance (under low-current test) varies with brand, age and use. Tenby 15A BS546 sockets, widely used in theatre installations, are perhaps the most variable of all.
 

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