Test To Measure Zs For Rcbo Final Circuit

[kme]

I think paddler DID intend to include HBC / HRC - as many of us would.

Each has its uses, as detriments. Part of "the trick" is being able to determine the best OCPD for the situation - which has been, in the past, fuses.

There isn`t a hard & fast rule for OCPD selection, as there isn`t for most of what we do - three guys might design a circuit with different cable install method & OCPD protection - doesn`t mean any are wrong - they`re just different.

 

[says-les]

Agreed

 

[steptoe]

Are they ones you've just installed - change supplier

If existing tell consumer you will fit hbc fuses AGAIN.  Why is the world off fuses?  Because of MCB profits? A fuse can hardly fail!

Let's campaign to ban breakers or to ban the test that's causing all this fuss, and fit fuses!  Ban the Bomb! Ban the Breaker! Hooray!

I think I'll send this to MEM etc.  They may be reading this I hope.  Big brother is watching remember.

BTW, Steptoe - what have they failed in? Timing or welding?

Blame the lack of training the consumer to use the button - we need a tele ad to show this!  The Fire Authority do plenty public awareness.  No one seems to mention fire risk as well as shock risk.

And how about that school that was struck by lightning and was badly fire damaged.

Did you know that John Caviezel when up on the cross filming the Passion, was struck by lightning - Zs must have been low in his case as he survived.  Mel Gibson reports that one of the crew were struck as well.

During the real rain storm when Caviezel was stripped almost naked, and meserably cold, he looked down.  All the crew had sou'westers on and he cried out, "Why have you forsaken me Lord" and Caviezel points out that his initials are J.C.  Mel Gibson nearly swallowed his fag when he was told that, after he accepted the part in the film.

Back to MCB's - ban them, I say!  This is only since I have listened to the evidence/experience from all you good guys out there doing a fine job without being appreciated I suspect.

Should we start a "sign here" to all sparks who are out there re this?

We have Gas Safe.  How is Electric Safe coming along?  Yet another training assessment issue for all.

both, mostly simply fail in NOT operating on the test button, that is usually after they have failed to trip on the trip test, as that is performed before pushing the T button.

BTW, 90% I have tested on PV have simply failed on time, poor installation.

EDIT, welding, I have NO proof of this, I do not dismantle to scrutinise.

 

[says-les]

Just stripped an rcd pod from an assembled rcbo memshield2 - piece of cake, which surprises me as the manufacturer's leaflet instructs not to attempt this! I am thankful I kept the original 3 parts that were required to be upgraded to suit the pods for each breaker.

I will swap over all Type C breakers to type B as you all advise - thanks to all.

 

[batty]

I have changed them in the past with no problems either. They just unclip.

 

[soulman]

Hi, I have briefly read through this thread & are a little confused.  Firstly I have been taught that if a 30ma rcd is in circuit zs now becomes 1667ohms.  I have also been taught that if Zs readings were slightly high on a TN installation but volt drop was o.k. changing the breaker from a type c to a type b could solve the problem or by putting in an rcbo/rcd into the circuit. TT installs usually rely on an rcd as ELI is normally too high to provide the required disconnection.  Does this mean that TT installs are generally unsafe as they are reliant on the rcd device.  I would have thought that changing the c.b to an rcbo would be one answer to the question in the church.  The circuits would be then be relying on the rcd device for disconnection.  Or have I got this totally wrong??

 

[says-les]

You are in the same school of thought as I am.

Thanks for your interest,

Cheers

Les

Good you are live.

I couldn't get my head round why many sparks replying to this thread were under the impression that Zs still had to comply with the 0.58 Ohm figure for a 32A type C breaker despite the addition of the rcd pod.

Cheers

Les

The building was poorly designed and laid out electrically having the mains Dist Board at one end of the building with no sub-dist boards - hence giving rise to long cable runs to final circuits leading to higher than normal Zs readings.

 

[soulman]

I may be totally wrong les, but this is what I have been taught. 

 

[steptoe]

using an RCD to hide or cover up a poor design would be a hard case to win in any court in this land,

especially when a simpler better compliant solution is available by simply fitting the correct type of MCB with a higher max Zs.

if a TN requires an RCD to comply with a Zs then there is obviously an inherent design flaw,

TT by its very nature requires the use of RCDs from the design stage onwards,

therein lies the difference,

can we all grasp the inherent different characteristics of TT and TN systems?

this is fundamental as to why simply using an RCD is not IMHO a valid solution.

 

[soulman]

Steps I appreciate what your saying, but this guy is not trying to hide or mask a poor design.  He is trying to improve it, yes he could change the breakers for type b & this would be the most cost effective solution.  But why not the rcbo offering additional protection to the user and allowing a higher ELI? 

 

[says-les]

You took the words right out of my mouth, Soulman.

 

[says-les]

I was asked to go over the 12 page EICR with the Trustees today which took me 2 hours of explanation,  The RCD issue is only one of the faults found by the contractor.  The building had no Main Bonding which has now been installed by me and this brought down all Zs readings but from the worst case from 0.9 ohm to 0.75 ohm.  The guys want it down to less than 0.58 ohm!.

 

[says-les]

I told the Church that if it was left to me I would rewire the lot after redesigning  the Installation.  The cost would probably be prohibitive.  The original building was done by DIYers and the only electrical person was an armature winder.

BTW Reg No. 411..4.4 answers all objections to me fitting the Pods. Steps, this would stand up in court I firmly believe.

 

[Steve3948]

Well in a court of law it would be asked if it complied with BS7671, well judge it's less than 1667 ohms which limits the touch voltage for the safety of life.

Case closed let's all go home and worry about mix or match Mcb's and come back next week because we have a case where Mr Mk Mcb had Mr Wylex bus-bar stuck up his backside which is a criminal offence in this land of stupidity.

 

[soulman]

Hi, I spoke an electrician friend of mine this morning he said I am wrong, He explained it is to do with short circuit line to neutral & told me to have a look at the tables of chapter 7 of the osg guide.  I have had a look & it does say NPsc  with a 30ma rcd.  So how does this work for TT.

Could someone please explain in detail, because I am now truly lost & late for work.

Cheers

 

[says-les]

To that electrician friend of soulman, what do you say to Reg 411.4.4?

 

[Steve3948]

Doing a loop test between L & N will give you the answer to that Soulman.

 

[soulman]

Hi says-les, He could not really explain it to me or tell why putting an rcd into the circuit would not be acceptable.

 

[ProDave]

We have been over this already.

You need to measure the L-N loop impedance of the final circuit to ensure the MCB will trip in time in the event of an OVERLOAD fault.

Most of the discussion on this thread seems to revolve around fitting an RCD "allows" you to have virtually any value of Zs and it will still trip in the event of an EARTH fault.  I blame the wording of the regs for placing too much emphasis on earth fault trip times only.

But fitting an RCD won't guarantee a trip in the event of a short circuit L-N which could well leave the cable overheating and catching fire before the MCB trips if the design is wrong. That's the point we are trying to argue needs proper testing to ensure it is safe. And in the event the loop impedance is too high, might mean you need to change the MCB rating to a different value or curve to ensure adequate disconnection times. 

 

[soulman]

This has drove me nut's for a couple of days.  It has finally sunk in, I think!!!  Pro Daves 12th post has it in a nutshell as an electrician explained to me this morning.  I have never been taught this at all.  Not even in the 2391.  Thank God for TEF & the clever chaps who help us and keep us on the right track.  I have just one more question regarding TT systems,  Zs is measured at the end of the circuit and usually quite high, and the reading recorded on test sheets, should I also be checking L-N loop impedance to ensure the protective device will operate in the event of a short circuit fault.  It seems as though the focus is only on EFLI & doesn't take into account short circuits.

Cheers