ECN Forum Forums General Topics of the Trade Non-US Electrical Systems & Trades wiring rules- installation design o.k.??

Amandaamisha,
Welcome to ECN!, always nice to have a new Member, especially from a place that we have no other ECN Members,just makes our membership base more diverse.
Ever have a problem Electrical?, just ask,
we are only too happy to help.

You have a feeder with a earth conductor but without earth-leakage sensing at the incoming side? I fail to see how this could develop into a phase-to-phase fault: It would if the cable melts, but is there really a risk/chance of this happening? I would have thought that a phase to earth fault on a TT system would have left the entire earthing system left floating at 240V.

Would it not be possible to install a time delay RCD at the beginning of the feeder? With a time delay, it will only trip on earth leakage in the feeder or in circuits without RCD protection. It won't trip if there is a non-delayed RCD downstreams between the fault and feeder, as the latter will trip before the delayed RCD unlatches.

Dear Trumpy,
Thanx for assuring me the support.
Frankly speaking - the orgin of the problem is as follows.
One silent night it so happened that in the switchbox(TP MCB +RCCB,30 mA) feeding to a 3 phase load there was a spark and simultaneously there was a blackout because of tripping of main breaker.On further investigation we found that :
1.Below the switchbox embedded in the wall was a loop-in box of only 3x3 size whereas the incoming feeder cable was 16 sqmm 4 core.
2.Needless to add the big size cable was all crumpled inside the small loopin box ,the cable was severely under TENSION and was rubbing against the male bush(GI) and over of period of time the cable insulation was getting damaged.
3.Now in the absence of earth leakage protection ,the insulation continued to get weakened /skinned and finally the fault developed into a phase to phase fault.
4.This accident has forced me to sit down and think about providing a earth fault protection at the mains incomer,but I need a mandatory regulation to prove my case (if I am right).
I am sure incidences of this sort happen once in a blue moon and is sure an installation mistake - cause I feel,had they used a bigger size loop-in box the cable would not have been in tension.I was intending for a earth leakage relay of multiple sensing current and time settings.

Please advice.

There is one way you could sort out the job, as i dont know your electrical regulations this could not come in line with your preset regulations,

Supply from PoCo to meter via Cutout, take your supply from your meter point to a TP/N isolator then to a time delay RCCB then to you Distribution board.

with the TP/N isolator this gives you a test and connection point for your load supply and metererd supply in and the RCCB (time delay) gives you the protection that you requested for the installation,

always ensure that your earth connections are true and tight and are within the regulations as to there physical size,

this should ensurew that the installation is safe.

And who installed the 16mm TP/N in such a small JB ??
Boy that must have been tight. !!!

I hope this answers your questions, if you need any other information come and visit us again, its nice to have someone from so far but to feel so near.

John H

Dear Harrison,
As mentioned in my orginal post the cutout at my installation has a TP MCCB of ABB make into which one can incorporate a Earth leakage release of variable sensing current and time setting. BUT THE BIG QUESTION I AM REPEATEDLY POSTING IN THIS FORUM IS WHETHER THERE IS ANY MANDATORY REQUIREMENT TO VALIDATE (SAY NEC OR IEE RULES) THAT ONE SHOULD STRICTLY PROVIDE A EARTH LEAKAGE PROTECTION FOR SUCH INSTALLATION ??

Initially REDSY had stated NEC Article 230.95 ,but unfortunately I lost the touch with him.
Please help.

Amandaamisha.

There is a requirement under our IEE regulations for electrical installations for a TT supply to be protected at the main distribution point with a RCD (Residual Current Device) otherwise known as a GFI.

But only at the distribution board not at origin.

As I said previously if you wish to protect the incoming main cutouts, install a RCCB or you can install a time delay RCCB/RCD at this point.

What do your regulations state ?

We can only advise on what we know within our own countries, but it does appear that your installaltions would most probably come under the IEE regs, as you describe the types of installations we do day in and day out.

Again i hope this helps,

Also what is the layout of the incoming installation before it gets to the distribution board, this is how i would normally do it.

PoCo cutout, TP/N 25/35mm tails to
Meter meter to main isolator (100a TP/N) with HRC fuses.
25/35mm tails to time delay RCCB/RCD enclosure.
Then to main distribution board in either 25/35mm tails (if it is less than 2m away from cut and meter) or Steel Wire Armoured 4 core 35mm if over 2m.

No other connections will be made except within the distribution board, no take offs for other boards etc, sub feed from main bistribution board only.

I hope this helps, as before let us know if you need any further help. you can always e mail me on john@empireconstructions.com of jhatdts@aol.com or come on back here then we can all help.

bye for now.

John H

Hmm, if that first phase shorted to the box, then the box, and anything bonded to it would have been hot at 240V to ground until the second phase shorted and finally tripped the breaker.

John,
I think Amanda is looking for an actual IEE Reg. number to quote to the bosses. I loaned out my copy of BS7671 and haven't had it back yet (wearing one of my other hats the last couple of weeks!). Can you (or maybe David?) dig out the appropriate reference?

Amanda,
If whoever is in charge of your plant would indeed accept the American NEC, then article 230.95 that Redsy mentioned states:
There's more to 230.95, but that's the basis of it. A British-style 240/415V service would certainly qualify under the voltage requirements stated here, so if the main disconnect is fused for, or could be fitted with HRC fuses rated at 1000A or more, then you could argue for a GFI/RCCB under 230.95. (At least that's how I interpret it.)

But as I said, that would all depend upon whether Kuwait/your boss would accept the NEC anyway. The American NEC doesn't cater for TT earthing systems, because they aren't used there (the U.S. uses TN-C-S).

Paul,

going to the office to dig out my copy of the regs.; back in a bit;

Found it;

IEE regulation 531-01-01 part of 531-01 Overcurrent Protective devices

531-01-01 states the following;

for a TN or TT system every overcurrent protective device which is to be used also for protection against electric shock (indirect Contact) shall be selected so that its operating time is;

(i) appropriat to the value of fault current that would flow in the event of a fault of negligible impedance between a phase conductor and exposed conductive parts, so that the persissable final temperature of the phase conductor and the associatted protective conductor is not exceeded ( see regulation 543-01-01)and

(ii) appropriate for compliance with the requirements of regulation 413-02-04

regulation 413 Protection Against Indirect contact

413-02 Protection by earthed equiponential bonding and automatic disconnection of supply ;

TT systems regulation 413-02-18

As follows;
413-02-18; TT system
EVERY exposed conductive part which is to be protected by a single protective device shall be connected, via the main earthing terminal, to a common earth electrode. however if serveral protective devices are in series, the exposed conductive parts may be connected to sepaerate earth electrodes corresponding to each protective device,
413-02-19
one or more of the following types of protective device SHALL be used, the former being prefered.

(i) A residual current device
(ii) an overcurrent protecive device

413-02-20
the following condition shall be forfilled for each circuit.

Ra Ia greater than 50v
where;
Ra is the sum of the resistance of the earth electrode and the protective conductor(s) connecting it to the exposed conductive part.
Ia is the current causing the automatic operation of the protective device within 5 sec
when the protective device is a reidual current device, Ia is the rated residual operating carrent In;


This is the regulation for the overcurrent protection of the installation as stated within the Institute of Electrical Engineers regulations for electrical installations 16th edition (otherwise known as BS7671)

I hope this helps, and yes you do have top protect the distibution board ONLY not the cut out.

Bye for now.

I am now off to the pub for a well earned beer, it is not aften i have to actually refer direct to the book for divine guidence, (unless i have a snotty client who thinks they know more than i do, and if they do why are they employing our company to do the work??)

bye for now.

John H
Electrical/Security Copnsultant Engineer for Empire Construction & Developments LTD

413-02-20 -- That's the one I was looking for, thanks John. (Must get my copy back!)

Amanda,
This is basically saying that if a phase conductor were to short directly to any exposed (earthed) metalwork, then the protective device must open the circuit within 5 seconds if the voltage on the metal exceeds 50V.

This isn't going to happen on a TT installation unless you install an RCD/GFI at the main panel.

Is this going to satisfy the boss of the plant?