High Zs on cct

[Deleted member 9648]

my 1st post on page 1 mentioned this and also the chance of contacting the manufacturer to see if he would allow a higher zs
the thread just went a little off piste

The circuit is protected by an rcbo.....the max Zs for the mcb part of the rcbo is rendered irrelevant by the rcd part of the rcbo which will ensure the disconnection time is achieved at a Zs of up to 1667 ohms.

 

[Mr Mark Sparks]

The circuit is protected by an rcbo.....the max Zs for the mcb part of the rcbo is rendered irrelevant by the rcd part of the rcbo which will ensure the disconnection time is achieved at a Zs of up to 1667 ohms.

so with the danger of going over this again

you dont need to worry what the zs is if the circuit is rcd protected

i am not starting an arguement just playing devils!

 

[Deleted member 9648]

Of course the zs is relevant....but any problems with a very high zs on a TN system should be picked up and rectified before the zs is even measured...high ze or R1R2 for instance.
In the case discussed here the measured zs is only a tiny fraction of an ohm above the max zs for the mcb to meet the required disconnection time.If the mcb was the only means of meeting the disconnection time it would need to be changed,but in this instance the rcd element of the rcbo ensures the disconnection time will be met up to a zs of 1667 ohms.
For the life of me I cannot see the logic of changing the mcb type to achieve a disconnection time which is already met by the existing protective device.

 

[Mr Mark Sparks]

Of course the zs is relevant....but any problems with a very high zs on a TN system should be picked up and rectified before the zs is even measured...high ze or R1R2 for instance.
In the case discussed here the measured zs is only a tiny fraction of an ohm above the max zs for the mcb to meet the required disconnection time.If the mcb was the only means of meeting the disconnection time it would need to be changed,but in this instance the rcd element of the rcbo ensures the disconnection time will be met up to a zs of 1667 ohms.
For the life of me I cannot see the logic of changing the mcb type to achieve a disconnection time which is already met by the existing protective device.

i fully agree and had this situation myself with a ring i modified and had to remove the type b mcb to fit a type c rcbo (merlin) the max recorded zs was .82 so only a little over i just noted it with the same reason you listed above

its just interesting how people interperate things

 

[devon1]

If it was a TT system, then I would be attempting to get all readings below 100 ohms no matter what the front end RCD is. If it was a TN system, then I would completely disregard all maximum Zs's relating to RCD's and just work to the max Zs's from MCB's as you would.

I'm on a job with TN-S, RCD 30Ma, 16Amp MCB.

The engineer on site is working to the TT Zs of 1667 ohms max. (i don't think that is correct).
The measured Zs is 8 ohms on that circuit and 80 ohms on others.

The Regs give approx Zs of 2 ohms max for those cicuits on TN-S . so those circuits fail the TN-S system max Zs.
Does the fact that they have RCDs make it safe?

 

[pevvers]

If it's a decent TN-S system, why are the Zs so high? 80 ohms is a ridiculous value for this type of system, and if common sense prevails then investigative works should be put into action to find out why it's so high with a TN type supply.

 

[devon1]

on that supply the Ze was .35 ohms, so it was ok.
what we are up against is the engineer thinks everything is ok if it's below 1667 ohms.
yes i think we definitely need to investigate the 80 ohms measured circuits and other like that, but the engineer will not put labour on it, just passes them.
by the way the circuits are for temporary supplies and the main distribution supplies with the RCDs are only fully tested every 12 months and functional tested every 3 months. I think they should be fully tested every 3 months.

 

[pevvers]

So, from board to measurement point of the 80 ohms circuit, what is the distance and cable size and type? Approximate of course.

 

[devon1]

i wasn't testing that cable myself, it was a 3 phase cable, one of the phases had the high 80 ohms. cable size is 4mm and long runs, type is SY, perhaps 100 mtrs to a distribution board. Zs from the distribution board ok at something similar to .35 ohms. then another 125 mtr run in 125mm cable to the supply. Ze .35 ohms at the supply

 

[pevvers]

i wasn't testing that cable myself, it was a 3 phase cable, one of the phases had the high 80 ohms. cable size is 4mm and long runs, type is SY, perhaps 100 mtrs to a distribution board. Zs from the distribution board ok at something similar to .35 ohms. then another 125 mtr run in 125mm cable to the supply. Ze .35 ohms at the supply

So only one phase on a three phase supply has a high Zs reading, and the other 2 are okay?

 

[devon1]

the others have readings of 4 ohms, which is about the max of approx 2 ohms in the Regs for MCBs on the TN-S system.
so they would fail if you went by the regs.
but the engineer is saying that anything below 1500 ohms approx passes.

I am working on a very large site in England. The engineers are saying it doesn't come under the IEE regs, but obviously it does come under the health ad safety at work act.
The supply is TN-S with permanent distribution units with RCDs. They supply temporary leads, lighting power etc.

 

[Guest123]

I'm sorry, but from what I can make out form your querey.......your engineer chappy is a plank!

You state you have a Ze of 0.35 on a TN-S system and a further Zdb of around the same, yet some of the circuits are measuring out at 80 ohms!!!!!!

Does he have any idea how long a circuit would have to be to return a Zs reading that high with that associated Zdb???

But it's ok because theres an RCD on the circuit.....I've never heard such horse **** in all my life. There is obviously a serious issue there somewhere, one that needs addressing ASAP.


Ok just to satisfy my curiosity I've done a rough calc and in order to obtain a Zs of 80 ohms without any circuit faults for a 4mm+4mm SY cable and your Zdb of 0.35ish as you say, the length of run would have to be around 230M. Does this sound right as a length of run to you???


The only thing that springs to mind is that the engineer was aware of this but rather than sort out what is obviously a problem he took the easy way out and gave additional protection via a 30mA RCD. A cop out in my book in this instance.

 

[pevvers]

Ok just to satisfy my curiosity I've done a rough calc and in order to obtain a Zs of 80 ohms without any circuit faults for a 4mm+4mm SY cable and your Zdb of 0.35ish as you say, the length of run would have to be around 230M. Does this sound right as a length of run to you???

But that's only for one phase, the other two are 4 ohms! I suspect a high resistance joint or damaged cable myself.

The only thing that springs to mind is that the engineer was aware of this but rather than sort out what is obviously a problem he took the easy way out and gave additional protection via a 30mA RCD. A cop out in my book in this instance.

I think we've had this discussion a few times before too, bad circuit design for the 4 ohm bit, and a bit of ooh b*gger, I had better stick an RCD on that.

Poor work indeed.

 

[Guest123]

But that's only for one phase, the other two are 4 ohms! I suspect a high resistance joint or damaged cable myself.

Thats the point I was making.....for the other 2 to be 4ohms and 1 @ 80ohms, obviously an issue there somewhere as no-one in their right mind would install a 4mm radial @ 230M.

Poor work indeed.

An engineer indeed

 

[devon1]

But it's ok because theres an RCD on the circuit.....I've never heard such horse **** in all my life. There is obviously a serious issue there somewhere, one that needs addressing ASAP.


Ok just to satisfy my curiosity I've done a rough calc and in order to obtain a Zs of 80 ohms without any circuit faults for a 4mm+4mm SY cable and your Zdb of 0.35ish as you say, the length of run would have to be around 230M. Does this sound right as a length of run to you???


The only thing that springs to mind is that the engineer was aware of this but rather than sort out what is obviously a problem he took the easy way out and gave additional protection via a 30mA RCD. A cop out in my book in this instance.

Some of the temporary leads etc maybe 230 mtrs long , due to other trades plugging in extension leads upon extension leads. But on the circuit mentioned it was 'designed' to be that long.
He is also quoting a max volt drop of 6% on lighting (not 3% as in the regs) from local MV supply.
Yes all these bad reading should be investigated, but i think the engineer (apart from not knowing what he's doing) doesn't have enough labour to assign to those tasks. small budget, very large site.

The RCDs are permanently fitted to the main permanent distribution units.

 

[Guest123]

At the end of the day, no matter how long a circuit was all 3 phases should have a Zs almost identical to one another....give or take a 0.01-2.

For one 2 to be @ 4ohms and 1 to be @ 80ohms, even my Nan would investigate that!!!!

 

[devon1]

The only thing that springs to mind is that the engineer was aware of this but rather than sort out what is obviously a problem he took the easy way out and gave additional protection via a 30mA RCD. A cop out in my book in this instance.

yes probably a cop out.
all the sparks on the section are not happy with it. they are all praying no accidents will happen.

 

[pevvers]

An engineer indeed

Yeah, sorry Lenny, didn't know you were getting at that point...

 

[devon1]

Lenny, the site has THOUSANDS of leads with faults like those!

 

[Guest123]

I dont think I want to know anymore about that site, I have enough trouble sleeping as it is.

 

[pevvers]

Lenny, the site has THOUSANDS of leads with faults like those!

I would take it higher up the chain if I were you, bypassing this "engineer".

I'm not sure if you're an employee, or a subby, but if you're an employee, be aware of duties placed onto you by the Management of Health and Safety at Work Act 1999 -

(2) Every employee shall inform his employer or any other employee of that employer with specific responsibility for the health and safety of his fellow employees -

(a) of any work situation which a person with the first-mentioned employee's training and instruction would reasonably consider represented a serious and immediate danger to health and safety


Just some food for thought and statutory regulations are pretty good when persuading. I would class this defect as a serious and immediate danger to health and safety.

 

[devon1]

I dont think I want to know anymore about that site, I have enough trouble sleeping as it is.

yes, nightmare! the trouble is it's a deep routed long standing story of neglect and every person from sparks upwards is in a failing system, nobody seems to be able to control it.
from experience if you go to the HSE, because they are not (usually) electrically trained they pass it back to the engineer and it all goes around in a circle again.
cheers anyway!

 

[pevvers]

yes, nightmare! the trouble is it's a deep routed long standing story of neglect and every person from sparks upwards is in a failing system, nobody seems to be able to control it.
from experience if you go to the HSE, because they are not (usually) electrically trained they pass it back to the engineer and it all goes around in a circle again.
cheers anyway!

There's no need to go to the HSE, if it's a construction site then there will be someone in charge of the health and safety on the site. At least if you inform them then you have fulfilled your statutory duties and if something did happen you'll be in the clear.

 

[devon1]

Lenny & Pevvers, thanks guys,

I will go and speak with the senior H & S man, i have already met him when i did the induction a while ago and got on with him very well.
i'll have a confidential meeting with him. i do indeed owe it to those poor unknowing workers of the dangers they are working in.

Lenny, i hope you can sleep better now!