Bringing Zs value down

[onions1066]

Just a quick question, need to add some lighting to an existing circuit, during Pre tests found zs value exceeds maximum permitted, it is a long circuit to be fair around 200m, it is 1.5mm twin. It is on a 10amp type b at the minute cant drop it to a 6A due to the amount of fittings on the circuit. There is a sub board half way round the building in the lift motor room I have the choice of re feeding the second half from this board which is possible but the other problem is it has a 110v emergency back up supply to each fitting which is the same length. Would it be deemed acceptable to pick up a 4mm earth from the sub board half way round into the circuit to bring the zs down? I really can't think of another way around this unless anyone has an idea?

 

[Plonker 3]

Lift DBSs are generally only for use in any lift equipment so would check this before connecting anything to it, as they have there own regs.

200 meters that's a hell of a run, how much over the accepted value is it? Could you check all connections are tight, is every fitting over the acceptable value? If not could you not come off a fitting that has a acceptable Zs to feed your lights then highlight any other problems to the customer?

 

[GLENNSPARK]

Just a quick question, need to add some lighting to an existing circuit, during Pre tests found zs value exceeds maximum permitted, it is a long circuit to be fair around 200m, it is 1.5mm twin. It is on a 10amp type b at the minute cant drop it to a 6A due to the amount of fittings on the circuit. There is a sub board half way round the building in the lift motor room I have the choice of re feeding the second half from this board which is possible but the other problem is it has a 110v emergency back up supply to each fitting which is the same length. Would it be deemed acceptable to pick up a 4mm earth from the sub board half way round into the circuit to bring the zs down? I really can't think of another way around this unless anyone has an idea?

so...

how many points served on this 200m run of 1.5mm twin?

volt drop anyone?

 

[alanl]

Yes, volt drop will be the killer here.
At 200 metres and 10 amps and 1.5mm conductor it will not comply now, more figures are required to given an accurate figure.
I'd be looking at putting a new circuit in - do the figures, and 6mm may be required for that length, so it should be divided into separate circuits, so the load will allow the Vd to be smaller.

 

[Rockingit]

Yep....I think you've more issues here than just Zs! Is the 200m figure a guess or have you measured it? Do an R1,R2 test at the furthest point, then do the mOhm/m maths to work out exactly how long the run really is and start from there.

There are ways to overcome Zs requirements if you're 'just on the edge', but VD and poor design is a different matter.

 

[telectrix]

how far from supply to the 1st fitting?

 

[7029 dave]

Split the circuit, if you can.

 

[GLENNSPARK]

parrallel this one up....or even make it a ring....

 

[Engineer54]

Yep....I think you've more issues here than just Zs! Is the 200m figure a guess or have you measured it? Do an R1,R2 test at the furthest point, then do the mOhm/m maths to work out exactly how long the run really is and start from there.

There are ways to overcome Zs requirements if you're 'just on the edge', but VD and poor design is a different matter.

Totally agree, far more potential issues here than just Zs values out of compliance. Having a central battery 110V backup emergency lighting system (which is what it sounds like) is only going to add to these issues, ...but big time!!

 

[baazab]

what is the zs value for that circuit ,

 

[onions1066]

Sorry for the delay guys I did respond earlier but it didn't post for some reason, I was unable to disconnect to do an r1+r2 so could only get the zs value which was 7.40. Way over I know. The 110v side has not been tested but is the same length so assume there will be similar readings until I can prove otherwise. . It is a p shaped building with only 1 way round there is no way back from the last fitting to the board. The 200 meters has not been measured but is pretty much there abouts as it has been paced out. Distance from the board to 1st fitting is about 15 meters. Engineer 54 yes it is a central battery backup. I am hopefully there tomorrow for a bit so I can get a true value and have been asked to price up different ways of overcoming the problems I really can't see anything other than rewire them and do it properly. Probably tempted to go for self contained fittings instead of central battery as the fittings are crazy money.

 

[Richard Burns]

You could run a say 4 / 6 mm cable as far as possible round the building and then intercept the lighting circuit at that point.
Then you would have two smaller (6A) circuits, the longer of which would be supplied with the thicker cable to drop the Zs and the volt drop will be lower and having two 6A breakers at origin will mean Zs will comply.
It may even have been a 6A circuit to start with and the MCB was uprated at some time without any further testing.
This would resolve a lot of the problems, however the 110V supply will be almost impossible to resolve.

 

[GLENNSPARK]

You could run a say 4 / 6 mm cable as far as possible round the building and then intercept the lighting circuit at that point.
Then you would have two smaller (6A) circuits, the longer of which would be supplied with the thicker cable to drop the Zs and the volt drop will be lower and having two 6A breakers at origin will mean Zs will comply.
It may even have been a 6A circuit to start with and the MCB was uprated at some time without any further testing.
This would resolve a lot of the problems, however the 110V supply will be almost impossible to resolve.

you mean `uprated` without any further thought to how potentially dangerous it is to alter the characteristics of the circuit without first testing it...and being competant enough to be able to interpret the values obtained...

 

[onions1066]

you mean `uprated` without any further thought to how potentially dangerous it is to alter the characteristics of the circuit without first testing it...and being competant enough to be able to interpret the values obtained...

That is exactly right. There has been a lot of alterations and no one has highlighted any problems. An eicr was carried out 18 months ago and conveniently it was just under the maximum permitted. What a difference 18 months makes.

 

[GLENNSPARK]

That is exactly right. There has been a lot of alterations and no one has highlighted any problems. An eicr was carried out 18 months ago and conveniently it was just under the maximum permitted. What a difference 18 months makes.

listen..

i was in the wholesalers..oh..a couple of weeks back now..

there was this fella in there asking for a D curve breaker that he was going to fit...caus the existing MCB kept letting go...it was feeding a commercial fridge at this kitchen and the chef was doing his nut over it...
so, i asked him what the Zs was...he didn`t know but continued down the route he had chosen anyway....
i explained to him that if it went to fault, someone got a belt off it and it was found out that disconnection times hadn`t been met then he would have a lot more to worry about than some arsy chef getting all over excited..

saw him a few days later where he explained to me that the MCB that kept letting go had been swapped for another one and it was the I N that was wrong for the max demand
apparantly someone had been in that board and needed the MCB for their own circuit they had pulled in...so had nicked the fridge MCB for their own work...replacing it with a 6B..

the moral here:....never take anything for granted when you go to carry out remedial

oh and the guy who planned to install a D curve MCB without any thoughts whatsoever was an `electrician` of something like 20 years standing..

well, it takes allsorts i suppose....