Zs on a ring question

[dickydodds]

This is an old house - 1930s onwards.

Doing my first certificate and have a question on my Zs Reading on a ring circuit on a type B 32A MCB BS 60898.

The measured Zs is 1.21 (my R1+R2 reading is 1.27) and the RCD trips at 43.6ms. (5x =13.9ms)

Now table 2D in the OSG says the max Zs allowed is 1.16 ohms and I know I should be looking at a trip time of less than 40ms.

Is this actually acceptable or do you have any suggestions on how I can get the Zs down?

The house is a standard detached house, not massive and this is on the downstairs ring circuit.

On a seperate note, the upstairs ring has a broken neutral and short of lifting all the floors in each room (which the customer deos not want me to do), I have no idea where it could be - the socket connections are fine but there are some spurs. I have connected this ring to a 16amp MCB, treating it as a radial circuit. Is this ok?
Thanks, Dicky

 

[bugsy]

1: has your tester been calibrated recently
2: what was the Ze
3:If your plugging your EL tester into the sockets have you tried pushing in and out a few times
4:You need to find the fault on the frist floor Isolate the faulty leg then you may treat as two radials
(better to investigate and find the fault though)

 

[Deleted member 9648]

If the circuit is prorected by a 30ma rcd then the max allowed zs is 1667 ohms....so you are well under with 1.21.
The max 40ms rcd trip times only applies to the 5x test...on 1x the max is 200ms so again no problem with your readings.
The break in N continuity should be found,although a 16/20a mcb is fine as a temp fix. Only way is to break the neutrals at all sockets and test each leg untill you find the break.......If the live and cpc conductors are intact despite what you say the most likely cause is a bad connection at a socket,clamped on insulation etc Re check all n connections very carefully before getting in too deep.

edit....note that the 1667 ohms maximum zs applies to all circuits/mcb ratings where they are protected by a 30ma rcd....saves looking up the max zs readings for each protective device when filling in the schedule .

 

[dickydodds]

1: has your tester been calibrated recently
2: what was the Ze
3:If your plugging your EL tester into the sockets have you tried pushing in and out a few times
4:You need to find the fault on the frist floor Isolate the faulty leg then you may treat as two radials
(better to investigate and find the fault though)

Thanks for your reply.

1 Brand new megger 1552 fully calibrated.
2 Ze = 0.21 ohms. Its a TNS system so is fine as its under 0.8 ohms
3 I have noticed a couple of sockets that are iffy when reading Zs and I do aim to replace them (the garage is especially bad and all need replacing)
4 That sounds a good idea. I would rather find the fault but the customer is not keen on my lifting all the flooring etc. Obviously I know I should try my best to find the problem

dicky

 

[Deleted member 9648]

My reply doesnt warrent a mention then?

 

[dickydodds]

My reply doesnt warrent a mention then?

Yes of course it deos, I value everyones response - its great you all take the time to do so. I was just in a hurry this morning!

You have certainly explained the 40ms trip time for me as it wasnt clear to me and the 1.6k allowed when on an RCD which I forgot about!

I will also do as you say and take all the sockets off and check them - its real world experiernce from the likes of you that help us newbies!

All the best and thanks again,

Dicky

 

[Deleted member 9648]

Thanks for that....I feel so much better!!!!!!!
Another way to locate the break is to link out one of the legs N-E at the db and then go round all the sockets with a test plug doing a N-E continuity check. Mark all the sockets that you get a reading on with a bit of tape and note which point has the highest reading. Now break the link on the first leg and link out the other leg and go and test the rest of the sockets that you havent marked and again note the point with the highest reading.The break will usually be between those two points....Although it only works on a proper ring...a lot of spurs will confuse things....it often enables the break to be located without disturbing all the sockets.

 

[Guest123]

Your R1+R2 does sound a little high for a standard house sized ring circuit.

Is it 2.5-1.0mm t+e or 2.5-1.5mm???

What were end to end resistances??? With that R1+R2 I get around 2.5ohms ish ???

 

[pevvers]

Your R1+R2 does sound a little high for a standard house sized ring circuit.

Is it 2.5-1.0mm t+e or 2.5-1.5mm???

What were end to end resistances??? With that R1+R2 I get around 2.5ohms ish ???

Yes, I was going to go along the same route. I really don't like the catch all of it doesn't matter about the Zs, as you've got a a maximum of 1667 ohms to play with. If you're above the max Zs, with a Ze that low, then whoever designed it made a lot of mistakes, as 1.27 ohms on an R1+R2 on a ring is pretty high.

My calcs work out at about 50m of 2.5/1.0 t+e, or 65m of 2.5/1.5...

 

[Guest123]

I really don't like the catch all of it doesn't matter about the Zs, as you've got a a maximum of 1667 ohms to play with. If you're above the max Zs, with a Ze that low, then whoever designed it made a lot of mistakes, as 1.27 ohms on an R1+R2 on a ring is pretty high.

100% my centiments also.

 

[solo2771]

Not sure if anyone said this already but here goes, check spurs for bad connections first if you can see any

 

[Deleted member 9648]

Take your point about the r1r2 being high for a ring,although it could be due to a long spur leg and 1.0mm cpc.
As suggested by lenny can we have the end to end readings?....might shed a bit more light on it.

 

[dickydodds]

Hi all.

I cant give you the end to end readings as I dont have them anymore - I can get them again though in a few days if you really want me to.

However, youre right about the cpc. 1 leg of the cable is 2.5/1.5 and the other is 2.5/1.0 - this might explain the higher readings. I thought this was a bit odd but I suppose it was a radial circuit then some time later someone made it a ring.

I dont believe there are any spurs but I havent had all the sockets off - I didnt see the need as the polarity checks out. Could a spur really affect the reading as the spur would be outside of the ring and the resistance test shouldnt be affected by this.

Cheers,

Dicky

 

[Doug]

Could a spur really affect the reading as the spur would be outside of the ring and the resistance test shouldn't be affected by this.

I thought that when Solo suggested checking spur connections he may have meant checking where the spurs are tapped into the ring. Getting three wires neatly into a terminal can be more fiddly than just the standard two. Also, faults on a circuit are more likely to be found where it has been modified, such as by adding spurs.

 

[pevvers]

I dont believe there are any spurs but I havent had all the sockets off - I didnt see the need as the polarity checks out.

You shouldn't need to take the sockets off if you have done the R1+R2 test the correct way, as with the figure of eight test it will pick up spurs without having to take anything off. That's the beauty of the test!