Ask The Trades - Print Page

Ask The Trades
https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Electrical Questions >> Ring Final - Multiple Loops https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1234437476 Message started by JabbaJaws on Feb 12^th, 2009, 11:17am

Title: Ring Final - Multiple Loops
Post by JabbaJaws on Feb 12^th, 2009, 11:17am

Hello all. I was in my electrical class last night and we were covering a section on inspecting and testing.

When we got to ring circuits, it was explained to us that multiple loops, within a ring can be dangerous, in case of a fault.

What l do not understand is why. The tutor was a little unsure himself, though he was completely honest, which l respect him more for, than to give us a duff answer, which some tutors do.

This started a debate where several students who agreed that it was dangerous, with others saying that it wasn't.

My questions:

Let's assume for arguments sake that we are dealing with a single ring circuit, protected by a 32amp breaker, operating under normal conditions. Along comes joe bloggs the DIY enthusiast and adds a double socket, but instead of spurring it, he splits the ring into two.

What effect would making an existing ring into two rings have on the circuit?

Would this in effect allow the 32A breaker to operate faster?

Is it actually dangerous to do this? Or merely not recommended?

Are there any other factors that l may have missed?

I would appreciate any help with as much detail as possible, from people who are in the trade, and know their stuff. Thanks, JabbaJaws... :-?

Title: Re: Ring Final - Multiple Loops
Post by Zambezi on Feb 12^th, 2009, 12:39pm

I am not sure what you mean JJ?
If he opens/splits the ring it will be 2 radial circuits being fed from a 32A breaker. The trouble with this is that the cable is rated bellow 32A (there are a number of factors to consider, like length of run, installation method, ambient temps, covered in thermal insulation etc), so your breaker may not trip before the cable has failed (melted insulation, fire etc).

Title: Re: Ring Final - Multiple Loops
Post by JabbaJaws on Feb 12^th, 2009, 1:10pm

Thanks for your reply Zambezi. I have just read my post and it does seem a little bit confusing.

I didn't mean to say "split" the ring.

What l mean to say was 'create a parallel path across the centre of the existing ring, so that two loops would occur on the same circuit.'

I hope that makes it a little clearer for you to understand...

Title: Re: Ring Final - Multiple Loops
Post by Goodsparks on Feb 12^th, 2009, 2:42pm

Hmmm...

Well testing would be tricky, end to end would not show a loss of continuity unless both parts of the ring were broken.

Given that it would be near on impossible to test and equally difficult to detect that it had been done (without removing every S/O) I would say that it was dangerous.

Should not be a problem now though given that domestic work is only carried out by competent people ;D

Paul

Title: Re: Ring Final - Multiple Loops
Post by Y3 on Feb 12^th, 2009, 9:17pm

wrote on Feb 12^th, 2009, 11:17am:

He means a figure of 8, and thats why you cross connect the line & neutrals and line & cpc's when testing a ring, when you take a resistance at each socket, this helps you find spurs and figure of 8 connections within a ring, as the readings should be the same at every socket, I think there is a +/- 0.05 tolerance between the readings (actually, I think thats with the open ended tests). Cross connections and spurs are significantly higher. A figure of 8 is dangerous because it will look as if you have a ring when testing (open ends), even though there is a break somewhere, which means certain parts of the cable will be overloaded, causing fires etc... This is why you cross connect. When you have an installation wired in singles it can appear to be impossible to do as you dont know which leg is which, what you do is cross connect as normal, if the reading at the socket rises as you go towards the middle, then you know you have connected them wrong, so cross connect again but with the other earth or neutral, depending on what cross connect test your doing at the time, the readings should almost be the same at each socket.

Title: Re: Ring Final - Multiple Loops
Post by The_Trician on Feb 12^th, 2009, 10:07pm

Given the various ages and conditions of existing installations when inspecting & testing, plus the vagaries of test prod /croc contact resistance, a difference between skts of 00.5 ohms or more is quite common, even on new ccts which have been correctly wired.

Had this today. Did a ring cct - all brand new skts and brand new cable.

Max measured difference? 0.8 ohms, the sockets are double pole switched and brand new!

As Paul says, it would be nigh on impossible to pick up a bridged Ring final unless you pulled everything to pieces and tested line by line.

TT

Title: Re: Ring Final - Multiple Loops
Post by Zambezi on Feb 12^th, 2009, 10:47pm

I have just spent the last 10 minutes drawing circuit diagrams trying to figure out how you would tell if a ring has been bridged (if the 2 sections of the ring that are in parallel are of a similar length). I have drawn a blank...it has been a while since college so that probably does not help!
As TT says you would need to separate all the tails at each socket outlet, by which time you probably would have spotted the bridge.

As far as it being dangerous: you could have 3 live tails at a socket outlet, when you only expect 2. You could disconnect what you think is a spur only to find that it is still live. Both situations are easily overcome by using your tester correctly before carrying out any work. Or do as I do, treat every circuit as live even if you know it is not.

Title: Re: Ring Final - Multiple Loops
Post by Y3 on Feb 12^th, 2009, 11:34pm

Im just telling him how it is, it works for me and thats how it should be done. If you looked in the brackets that i had written in, you would notice that I meant 0.05 to be aimed at the open ended resistances of the line and neutral conductors, thats the tolerances between the two, the cpc should be 1.67 times higher than the line or neutrals, but parallel earth paths can obscure the cpc's readings. If you go in an existing property and do these tests, you will find spurs and possible cross connections if they are there and if so, you can put them right. The tests are in place to find these issues, im sure they are not always 100% but thats why they are there. I did these tests just today on a ring we newly installed which had one spur off it. All the sockets on the ring were almost all exactly the same, the spur was considerably higher.

Title: Re: Ring Final - Multiple Loops
Post by The_Trician on Feb 13^th, 2009, 12:39am

Stick to your guns mate - I admire you for that. If it works for you then that's good.

Have a go at an old installation, where the socket outlets have been correctly wired, but where the mechanical fit between your tester plug and the existing old, worn, and in some cases corroded socket terminals comes into play.

In many cases you'll get a much higher resistance than 1 ohm between any 2 skts.

Don't take the result as gospel - just check the terminal connections for tightness, and plug/unplug your test plug, and switch the socket rocker switch on and off a few times, and you'll see an immediate change in test values.

And yes, the cct has been correctly wired in most cases.

Sometimes, dismantling is the only way to test conclusively one way or the other.

Even a slight squeeze on the croc clips, or even a good poke with the test prod can change the result significantly.

TT

Title: Re: Ring Final - Multiple Loops
Post by LSpark on Feb 13^th, 2009, 12:49am

wrote on Feb 12^th, 2009, 2:42pm:

Should not be a problem now though given that domestic work is only carried out by competent people ;D

Well said Paul ;D

Carl, I can tell you've been swatting up on the testing stuff, and good on you. But you will find you'll need to diversify on occasion in practice, and not just to get around high resistance connections, which ofcourse is the problem when your only testing at low current and voltage. Bring on the 25A bond test!

Title: Re: Ring Final - Multiple Loops
Post by The_Trician on Feb 13^th, 2009, 12:55am

One caveat - be careful what you wish for.

Would you expect for example, a 5 day wonder Part P'eed kitchen fitter to have the requisite knowledge and experience to diagnose something such as a bridged Ring Final?

Remember, under the eyes of the law, and his scheme provider, he is classed as 'competent'.

TT

Title: Re: Ring Final - Multiple Loops
Post by JabbaJaws on Feb 13^th, 2009, 10:32am

As usual, thanks for all your much valued input guys. This is the sort of feedback l need, from actual sparkys who know what their doing. I'll be back soon with more questions.

Kind Regards

JabbaJaws... ;)

Title: Re: Ring Final - Multiple Loops
Post by Y3 on Feb 13^th, 2009, 8:06pm

LSpark wrote on Feb 13^th, 2009, 12:49am:

wrote on Feb 12^th, 2009, 2:42pm:

Should not be a problem now though given that domestic work is only carried out by competent people ;D

I understand everybody's comments and have taken them on board, tis true, what you say TT, there are factors involved that affect the readings in the real world and not on a board in college.

Whats a 25a bond test?

Title: Re: Ring Final - Multiple Loops
Post by LSpark on Feb 14^th, 2009, 12:37am

wrote on Feb 13^th, 2009, 8:06pm:

Whats a 25a bond test?

A test perfomed when carrying out portable appliance testing, not relavent to fixed wiring tests (generaly).

Title: Re: Ring Final - Multiple Loops
Post by ChubbyPhaseWire on Feb 14^th, 2009, 9:11am

LSpark wrote on Feb 14^th, 2009, 12:37am:

wrote on Feb 13^th, 2009, 8:06pm:

Whats a 25a bond test?

A test perfomed when carrying out portable appliance testing, not relavent to fixed wiring tests (generaly).

Take a look at Reg 507(b) page 72 of the 13th Edition 1962 Amendments. :)

The tests are preferably made by a.c. of mains frequency,at currents of the order of 1.5 times the rated current of the circuit, although currents of 25 amperes need not be exceeded. ;)

Title: Re: Ring Final - Multiple Loops
Post by The_Trician on Feb 14^th, 2009, 10:07am

Conduit testers anyone?

TT

Title: Re: Ring Final - Multiple Loops
Post by Y3 on Feb 14^th, 2009, 1:31pm

I dont own the 13th edition, way before i was even thought about! Somebody i worked with had it though and that was when we were working to the 16th, he would consult his 13th ;D

Title: Re: Ring Final - Multiple Loops
Post by LSpark on Feb 15^th, 2009, 2:27pm

I always knew he was stuck in the past ;D

Title: Re: Ring Final - Multiple Loops
Post by Y3 on Feb 15^th, 2009, 6:40pm

LSpark wrote on Feb 15^th, 2009, 2:27pm:

I always knew he was stuck in the past ;D

Who?

Title: Re: Ring Final - Multiple Loops
Post by Zambezi on Feb 15^th, 2009, 7:11pm

wrote on Feb 15^th, 2009, 6:40pm:

LSpark wrote on Feb 15^th, 2009, 2:27pm:

I always knew he was stuck in the past ;D

Who?

Chubby.

When I started reading his post I almost fell off my chair, I thought he had finally posted something useful, I should have know better!

Title: Re: Ring Final - Multiple Loops
Post by LSpark on Feb 15^th, 2009, 7:50pm

Indeed!.. we can only live in hope ;D

Title: Re: Ring Final - Multiple Loops
Post by dingbat on Feb 17^th, 2009, 1:05pm

wrote on Feb 12^th, 2009, 11:17am:

When we got to ring circuits, it was explained to us that multiple loops, within a ring can be dangerous, in case of a fault.

What l do not understand is why...

This started a debate where several students who agreed that it was dangerous, with others saying that it wasn't.

My questions:

What effect would making an existing ring into two rings have on the circuit?

The only actual danger with interconnections (bridged rings) is the masking of discontinuities - that is, end-to-end resistances appear to confirm the existnce of an intact ring.

Quote:

Would this in effect allow the 32A breaker to operate faster?

No.

Quote:

Is it actually dangerous to do this? Or merely not recommended?

Not recommended - this is not how rings are configured.

TT is absolutely right about the effects of wear and tear on ring circuit test results.

There is very rarely any defensible justification for using rings.

Title: Re: Ring Final - Multiple Loops
Post by ChubbyPhaseWire on Feb 17^th, 2009, 4:46pm

wrote on Feb 15^th, 2009, 7:11pm:

wrote on Feb 15^th, 2009, 6:40pm:

LSpark wrote on Feb 15^th, 2009, 2:27pm:

I always knew he was stuck in the past ;D

Who?

Chubby.

When I started reading his post I almost fell off my chair, I thought he had finally posted something useful, I should have know better!

;)

Title: Re: Ring Final - Multiple Loops
Post by plugwash on Feb 19^th, 2009, 1:53am

Afaict there are two issues with a "figure eight ring"

The first as mentioned is it is very hard to test
Also depending on the exact shape of the ring it may introduce major balance problems