Advice please

Hi I just recently passed my testing and inspection exam and just have a question when testing a circuit.

My question is if I install a short Ring with 4 fused switches next to the DB and then from each fused switch I run a 50m cable to a double socket, when I come to test for Zs do i treat my new ring as a separate circuit from the 4 50m spurs or do i treat the circuit as a whole and take my highest reading of r1 and r2 and add it to my Ze at the DB. Also if I treat them as separate would I treat the spurs as minor works. Thanks.

I’m sure there will be different views on this but if your original ring is 4 x fused spurs, then that’s all you can test.
 

If you later add load cables of 50m then each could be covered by a MWC .... however isn’t volt drop going to be an issue? 
 

Thanks Murdoch and yes voltage drop could be an issue another equation to work out lol.

Mell said:

Thanks Murdoch and yes voltage drop WILL be an issue another equation to work out lol.

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corrected that for you

Lol I stand corrected ty once again 

Just saying,  I would not design a circuit like that in the first place.

I'd test it all as one, and I'd make a note in the comments section that the R1R2 and Zs values are higher than the end to ends would suggest because of the long spurs. The only time I'd be tempted to test each as separate (which, assuming you are filling out an EIC for the ring would need an extra continuation sheet for each fused connection unit) would be if the Zs was too high for the breaker but low enough for the 13A fuse, and these days you might decide to say the RCD provides fault protection instead (its allowed on a TN system, just not the best practice in the world!)

As long as only one socket per leg you could probably even justify it without the spurs, just a joint. ****ty practice but you could..... some council houses were wired this way 'spider wiring' we termed it

As ProDave alludes to, just because something can be argued to comply with the regulations, doesn't mean it is a good design practice unless there is a specific reason its been done that way, it doesn't mean sticking to standard cirucits all the time, its using engineering judgement to come up with a sensible solution to whats required, and maybe also giving thought to what might be required in future, because you might be the one to go back and change it

I suppose the obvious question is why such an odd design ...and what are on the end of the spurs

Hi guys the design is for stage lighting bars x 4 10m high with led stage lighting on them, converting the old system to led so where the DB board is I am going to take out the old system and put in 4 fused switches on a ring then from those 4 switches each cable will run to each lighting bar and the longest cable run is approximately 50m, the DB has 4 32a rcbos for the old system, I will utilise one of them for the ring, the reason for fused switches is to be able to turn the bar off instead of the customer going into the DB board to switch them off when not in use. 

How is the lighting controlled? DMX? What size are the cables running upto the bar?Assume they went to a dimmer Pack before?

You'd be much better with 4 16A radial circuits and just put four isolators at the board, a ring doesn't really fit here!

as above... stage lighting is a bit different from most stuff... at a guess you plan on using permanant supplies to the lights and control via DMX instead of dimmed / non dimmed supplies to individual lights?

Yes the lights are dmx controlled and the old stage lighting ran off dimmer packs which I'll remove and utilise existing cables to make a radial circuit to each lighting bar and convert one of the 15a sockets to a double 13a socket, I did think about a radial circuit to each bar with a fused switch, I was only going to create a ring so I could use one of the existing 32a rcbos in the DB.

I would run that as radials and not wee about with a ring, as for RCBO, does this installation require RCD protection - it's not domestic, and coud be risk assessed as not required??? I would have though highly sensitve RCd protection might be prone to tripping during a performance and therefore worth avoiding at all costs. 

I always protect with an rcd or rcbo and you only get a tripping on powering up the leds as there is an in rush but I get over that problem by using C rated breakers as the B breakers always trip on start up.

Mell said:

Yes the lights are dmx controlled and the old stage lighting ran off dimmer packs which I'll remove and utilise existing cables to make a radial circuit to each lighting bar and convert one of the 15a sockets to a double 13a socket, I did think about a radial circuit to each bar with a fused switch, I was only going to create a ring so I could use one of the existing 32a rcbos in the DB.

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You could still use your existing 32A RCBOs to supply 4x radials, if you are having fused switches within 3m of the RCBO...

As reg 433.2.2 allows the fusing down for reduced conductor size to be within 3m of the change, providing there are no branches off the circuit prior to the new fuse position...

So 32A RCBO at board...  <3m of 2.5 into a fused switch that provides the appropriate protection for your 50m radial run..

This gives added benefit that one RCBO tripping will not knock out all of your lights.

Guinness    

Thanks special location I did not realise this I will have a look at that reg, i was always taught that your breaker had to be less than or equal too you cable current capacity unless it was a ring main circuit hence why I was going to make a ring and use one 32a breaker on 2.5mm cable on on fused switches.

Thanks everyone for your advice and yes I am still a little green with testing and inspection only past exam couple of months ago so from time to time may ask stupid questions especially if I can't find the answer in the regs book you guys are my next best thing as I have no one else to turn too to ask.

Mell said:

Thanks everyone for your advice and yes I am still a little green with testing and inspection only past exam couple of months ago so from time to time may ask stupid questions especially if I can't find the answer in the regs book you guys are my next best thing as I have no one else to turn too to ask.

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There are very few sensible answers in the regs book and at each update the situation gets worse ............. then people wonder why short cuts are taken or simple poor design happens.

Problem is that its very much like the bible, i.e. written by many people with little or no coordination between them

If it was up to me I'd simplify the regs book and immediately separate it into 2 books - single phase and 3 phase. AND I'd improve some of the OSG misleading nonsense too

Murdoch said:

d immediately separate it into 2 books - single phase and 3 phase.

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why? cant see what difference it would make, you would end up with 2 book of mostly identical content

definitely needs simplified and some of the lettuced removed

Andy™ said:

why? cant see what difference it would make, you would end up with 2 book of mostly identical content

definitely needs simplified and some of the lettuced removed

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I agree in point with what both you and Murdoch say, I remember once when I must have been totally losing the plot and read a copy of the regs cover to cover, the number of items I found that contradicted each other were unbelievable, in one section it would say something like you must install so and so when fitting such and such, then later on it would say you must not install so and so when installing such and such, clearly different things written by different people who never got together and discussed it. Also the constant ammendments and updates don't help, not the least with the added cost of buying these updates. These days I'm fairly of the belief that the regulation of our industry is driven by manufacturers and the selling of their products has become more important than actual safety considerations, AFDD's being a prime example of this. We have no way of testing them 'on site' either to prove they work, or to demonstrate to the customer that this expensive piece of kit does actually do something. With an RCD/RCBO, we can test them with our meter, to either prove functionality or to demonstrate it to the customer that it will operate in a fault situation.

Also we need more control of I&T, now I'm not having a go at the OP, as he says, he's passed his exams, but clearly lacks the experience to understand how to do what he is trying to do, again as I say it isn't his fault, someone obviously told him he could do it, and no doubt made some money out of him too. Now as I am sure that the OP himself will admit, it isn't as easy as he was led to believe it is.

One of the biggest problems we seem to find, and we have discussed it here numerous times is I&T,  incorrect codings, things not coded that should be, things coded that shouldn't be, customers told they need a rewire when a bit of remedial work would suffice. Now I know that this is sometimes the result of unscrupulous so called electricians and sometimes as a result of a lack of knowledge on behalf of the tester.