Live Or Dead Testing - A Question For Members

[Sidewinder]

says-les,

The design calculations should have shown up that the Zs for the circuits would have been outside that allowable, thus, the design should have been changed to allow for this by changing the size of the supply cable at least.

If you have a TN-C-S/PME 3ph incoming supply I am surprised that your Zs is outside limits for a C32, @ 0.58 ohms measured.

I would have expected Ze to be around the 0.1 mark, that gives you 0.48 to play with, about 24m of 2.5/1.5 in radial config.

Go to 4/1.5 your up another 4m.

This would give you a ring length based on 2.5/1.5 @ 0.58 ohms & R1+R2 = (r1+r2)/4 of nearly 119m

By your description you have Vigi type units?

In which case why not swap the C32 for a B32?

This would give you 1.16 ohms measured, around 217m of ring length.

That way your Zs will meet the requirements for the MCB & the add on RCD pod will merely be for your additional protection to meet this requirement, not to overcome the poor design and installation practices?

Adding the RCD pods seems a lot more expensive than changing the C32 for a B32!

Taking list prices on Schneider kit even!

I need more insight into your thought process & the installation TBH to understand your point.

 

[apprentice87]

Hi Paul,

The Ze on my TNCS 3Ph supply is .28 Fairly dismal i thought, must be cos i am a long way from the DNO transformer presumably, or might it be that the 80 year old cable down my street is at its limits as it were. Wish mine was .1

Mind you, when they first installed it, L2 was .58 so i went and complained, especially as when i measured the same phase on a separate supply [in my house] it was much lower, despite the two service cable joints only being about a yard apart..... Turned out that they had forgotten to tighten the screws in the cutout..... Sooo, it might be worth checking Ze on all three phases to see if they are all the same...

john...

 

[Sidewinder]

john,

That is bad!

Mind IIRC, you are basically a 3ph "domestic" install?

Mind, even working on my figures with that, I feel you could get ADS with a B32.

I can't see why the OP has used C32's with an add on RCD pod, if they were single unit RCBO's then that is different as they are often only available in C curve.

Now trying to get my head around the reasoning for what has been done.

 

[apprentice87]

Hi Paul,

I know, i was a bit disappointed, especially as my tig pulls 70 odd amps, think of the resultant volt drop..

Coming back to the original thread though, i agree totally.. Why you would want to use "c" curve breakers in any sort of circuit in a church is beyond me, unless they have a 20 HP motor driving the organ!! Besides, why would you want a c32 to protect a ring of 13A sockets??

Only thing i can think, is when the original numpty installed it, the wholesalers that sold them the DB, only had "c" curve breakers in stock.

Sort of points out the difference between an installation that "works" and one that is safe..

john...

 

[Doc Hudson]

Hi Paul, I know, i was a bit disappointed, especially as my tig pulls 70 odd amps, think of the resultant volt drop.. Coming back to the original thread though, i agree totally.. Why you would want to use "c" curve breakers in any sort of circuit in a church is beyond me, unless they have a 20 HP motor driving the organ!! Besides, why would you want a c32 to protect a ring of 13A sockets?? Only thing i can think, is when the original numpty installed it, the wholesalers that sold them the DB, only had "c" curve breakers in stock. Sort of points out the difference between an installation that "works" and one that is safe.. john...

As the OP states that its a bit of a DIY botch up.  "The building had never been tested periodically nor was there any sign of design drawings, power distribution charts nor EIC. The building was by D-I-Y's and the only electrical person was an armature winder. This means that the whole design is bugs and the dist board is at one side of the building"  I would guess the DB may have been acquired by someone from work or a friend of a friend doing a cheap favor for the church restoration fund! 

Doc H.

 

[says-les]

You are so right, Doc H. The church have just brought in a small contractor to do an EICR so we'll see what that says. My problem is I joined this church a few years ago and have been finding fault after fault. I am retired spark who did the 17th Course and the Inspection &Testing three yrs ago. It's been a bone of contention this church installation with me for the few years now. No Pat testing done except what I check but not by a contractor. No Fore alarm yet they allow sleep-overs on occasion. Linked mains smoke detectors only. They have little funding. The building is hired to several outside organisations. I am at the point of walking away but determined to get them to toe the line as far as building maintenance is concerned. Help!

 

[apprentice87]

Hi Doc H and Says-les,

It is right what you say, i might well have been a load of stuff someone got from work!!

Says-les, you have got a problem there.. I know you are doing your best, but i think all you can do is advise them.. If they have not got the money to sort it out, well, not much that can be done... Big problem if they hire it out to outside people too. Think if there was a problem... Not sure what you should do..If you do your best, and then something goes wrong, the first thing tht will happen is that they will all say [when questioned] "Les did some wiring here!!" and then the insurers will come to talk to you..

On the other hand, you WILL be improving things, so i cannot see much wrong with that, you will leave it all in a better state than it started in.

If it were me, i think i would take before and after photos of everything i did, so at least if something DID go wrong, you could point out that it was not "your" bit!!

Difficult situation here...

What does everyone else think Les should do?????

john...

 

[says-les]

Thanks for that Doc. From the original post I am amazed at the volume of responses which is a good thing. It gives me an outlet among non-tech ppl.

 

[Sidewinder]

les,

What make is the DB?

Surely the funds can stretch to a couple of B32's?

Single pole are only a few quid each from the wholesalers.

Just make sure that the breakers & board are all the same make.

 

[Sidewinder]

BTW les,

We try to be helpful and constructive, though it helps if we have as much information as possible to formulate our answers.

There are several hundred+ members on here who give their own time for no reimbursement in an attempt to help answer questions such as yours.

We have members who have done short courses, have worked in the industry all their lives and are approaching retirement, we have associated skills, right through to members who do industrial and machinery type works, medical, and a whole raft of other skills, and some professionally qualified engineers with degrees in engineering, so a broad spectrum.

Stick around I'm sure we can keep on helping you if you want it and I'm sure you will actually have an awful lot to offer other members around here with your experience.

 

[SPECIAL LOCATION]

My problem is I joined this church a few years ago and have been finding fault after fault. I am retired spark who did the 17th Course and the Inspection &Testing three yrs ago. It's been a bone of contention this church installation with me for the few years now. No Pat testing done except what I check but not by a contractor. No Fore alarm yet they allow sleep-overs on occasion. Linked mains smoke detectors only. They have little funding. The building is hired to several outside organisations. I am at the point of walking away but determined to get them to toe the line as far as building maintenance is concerned. Help!

I think you need to take your electricians hat off a bit and just reassess exactly who's responsibilities are what with regard to sorting out the problems here...

1/ Are you or have you ever been paid as a formal job contract to undertake or manage any of the electrical work?

2/ Are you in any position of authority management legal responsibility toward the daily running of the church?

3/ Are you just a volunteer member offering a bit of advice and practical help in a non earning capacity? 

if you are 1 or 2..  obviously you do have a bit more responsibly to get things sorted safely...

but if you are 3/ you may be best just putting your concerns in writing to those that do have the responsibly

and let them deal with it...

Because any decisions will probably have to go through some sort of committee before any final decisions are taken and church committees can be about as fickle as they come...

may take years before anyone actually sees that there is an issue to resolve!

Is it your problem..  or someone else's problem?

if its someone else's it may be best to just let it go..

:C

 

[says-les]

Thanks, Sidewinder for your input. The DB is an Mem with 2xvertical columns of CBs and TP&N.

I am no. 3 in your last post.

 

[says-les]

Correction should have referred to Special Location in last part of last post. Ta Last Post.

 

[says-les]

I'll get it right yet, Ta Special Location.

 

[batty]

Memshield 2 breakers are around a ten to twelve pounds now they have moved onto memshield 3 so its not going to cost a fortune to put that bit right.

 

[says-les]

Let's forget the Church problem that I have walked into open eyed for a moment, and put the following thoughts to the members out there.

Start from first principles.

A Fuse or CB must clear a direct short from Line to Earth within 0.4 seconds in order to provide Shock Protection.

So Zs must be measured to check that sufficient fault current will flow and clear an earth fault within this time.

In addition, that fuse or cb must also provide overload AND short-circuit protection based upon the current rating of the cable on its load side.

We are not required to test that the fuse or cb will clear an overload or short-circuit but we must by observation check that the rating of the fuse is no greater than the rating of the protected cable, after any/all rating factors have been applied.

If an RCD is fitted as an add on pod to a CB then does this not make the duty of the CB for earth fault purposes redundant since the RCD part will trip the breaker within a very short time without quoting the test trip times measured by the RCD tester.  So is there any need to see that the Zs for that circuit is within the value required for the CB to clear the fault, had the RCD pod not been added.  My logic says no.  Some of the guys who responded to this post think Zs must still satisfy the max allowed value according to Table 41.3 remembering the 0.8 modifier for that table.

Now suppose the Zs measured for an EICR, which it must be to fill in the appropriate column in the Test Schedule, is found to be satisfying a Type-B but not a Type-C CB, then one solution is to replace the type-C with a Type-B.  Again my thinking tells me no need to do this, although it would solve the issue, but as long as there is an RCD pod fitted to satisfy any of the reasons given in the Regs that require an RCD (Bathroom lights, sockets wired in a non-metallic sheathed cable type and buried in plaster at a depth less than 50mm from the surface of walls, etc., or for any other additional protection deemed necessary such as in a nursery) then Zs can be up to, 1667 ohms, I think from memory. This comes from the Reg that states that the product of the trip current Ia x Zs must be less than 50 volts.

I wish I had been a rubbish collector! LOL

 

[ProDave]

If an RCD is fitted as an add on pod to a CB then does this not make the duty of the CB for earth fault purposes redundant since the RCD part will trip the breaker within a very short time without quoting the test trip times measured by the RCD tester.  So is there any need to see that the Zs for that circuit is within the value required for the CB to clear the fault, had the RCD pod not been added.  My logic says no.  Some of the guys who responded to this post think Zs must still satisfy the max allowed value according to Table 41.3 remembering the 0.8 modifier for that table.

Yes

Because not ALL faults are L-E.  A L-N fault would NOT trip the RCD.  So you still want the CB or fuse to break the circuit.

 

[says-les]

Thanks for that ProDave and thus do you take my point that we need not in that case be concerned nor mark the job down, if Zs is above the max Zs for the CB if it had been without the Pod? Or am I reading you the opposit from what you mean. Remember that a L-N fault is nout to do with Zs, and there is no need to consider disc time for a L-N fault?

 

[ProDave]

Thanks for that ProDave and thus do you take my point that we need not in that case be concerned nor mark the job down, if Zs is above the max Zs for the CB if it had been without the Pod?

I don't understand.

If Zs is too high for the CB or fuse used, it gets noted.

P.S how did this thread morph from a question about live testing, to concerns over the wiring in a church?  :C

 

[says-les]

Well this developed as some of the guys responded with questionable responses. It has opened a can of worms so to speak, not only about how we test but what are we accepting and what we are marking down. I modified my last reply but it may not have got to you in time for you to read b 4 you responded. Thanks any way.