Live Or Dead Testing - A Question For Members

[m4tty]

Sorry but what is a D63?

A D63 isn't a B32

D type breaker instead of B type normally used in domestics. D types are for heavy start up currents ie, big welders or X-ray machines. Used in big commercial and industrial normally rather than domestics. And the 63 is the amperage of the circuit breaker.

Hope this helps matey

 

[Technician]

Echoing Doc's point, from some of the posts I have read on this forum

a lot of DNO telepeople do not know the value of Ze anyway.  This makes

the requirement to measure it safely more imperative.

 

[says-les]

I take your point. Thanks

 

[says-les]

Thanks for that.

 

[mvkpe1]

As an additional thought to this debate, Where an installation is live and working and has essential services that cannot be turned off, then it can be impossible to safely measure Ze as you cannot remove the installations CPC and bonding connections to get a reading down only the suppliers earth facility (or the Ra of the rod). So if you cannot get an accurate Ze you certainly cannot add R1+R2 to a guessed Ze. In this case the most reliable and accurate verification that your max Zs limits are not exceeded is a live Zs test reading of the circuit  in question.

Doc H. 

As an additional additional thought you then have the option to add R1+R2 to the Ze as expressed by the SEB/SSB etc? normally 0.1Ohm, but I would always do a Zs in any case.  But if the installation is live and working and has essential services running you can't take R1+R2 readings unless you do a live loop test, so you might as well take a Zs at the same time.

 

[says-les]

All good points - thanks for your input. At least one spark said that the circuit protected by an RCD still has to meet the max Zs for that protective device. I disagree with this. What do you think?

 

[SPECIAL LOCATION]

All good points - thanks for your input. At least one spark said that the circuit protected by an RCD still has to meet the max Zs for that protective device. I disagree with this. What do you think?

What is regulation that you think suggests it is OK to exceed max Zs  if RCD protection is in place?

:C

 

[says-les]

The Reg that Says if max Zs for the cct cannot be achieved then an RCD satisfies that cct.

 

[says-les]

See 411.4.4

 

[Sidewinder]

It is "satisfactory", but not an excuse for poor basic design to achieve ADS.

Plus, you need to ensure that the L-N loop will disconnect also, which if you have TN-C-S & PME, & your Zs is excessive, then it may not.

You would need to double check/

 

[Technician]

Echo that Sidewinder;  Have seen some test results

where the PSSC was nearly half the PEFC.

 

[SPECIAL LOCATION]

See 411.4.4

?

Yours must be worded differently...

my 411.4.4 in BS7616 doesn't say Zs values may be ignored if RCD fitted..

What does yours say in 415.1.1   &   415.1.2

Mine says something along the lines of ..

use of RCD does not obviate the need to apply one of the protective measures specified in 411 thro 414

Either way you are asking the wrong question....

excluding the obvious limitations of TT systems..

What final circuits in an the average TN installation are you finding too difficult to be able to design & install

so that you cannot meet the Zs values for the protective device?

opcorn   

 

[Technician]

Note 1 in that reg;  Overcurrent device to be used.

Further, there is a specific reg stating that an RCD

cannot be used as the sole protective device.

Reg 415.1.2.

In level 3 there is a question about the use of an RCD

to reduce disconnection time.  But the answer does

not include removal of the overcurrent device.

 

[says-les]

That's correct. There needs to be sc and ef protection. Imagine visiting a job that has rcbo's. do we test for Zs and see that the max Zs is not exceeded? Or do we do the RCD test. I'm trying to say that the RCD pod added to the cb obviates the need to test for a max Zs figure since the RCD pod takes on the role of ef protection. In such case the overload part of the rcbo's protects from ol and sc. This, together with measured R1+R2 and measured Ze allows you to add and insert the Zs value.

 

[says-les]

In answer to Trailer Boy, the Reg 411.4.4 states that the following types of protective device may be used for fault protection:

(i) An overcurrent protective device

(ii) An RCD

Take e.g. an RCBO already fitted which is part of an installation you are testing.

What I'm saying is that if the RCD pod has been added because the final circuit's Zs is over the value for the type of cb from Table 41.3 (remembering that the values in that table have to be reduced to allow for fault temperature by multiplying the figures by 0.8, then you can forget the cb part of the RCBO as far as earth fault protection is concerned. The breaker has to be tested as for any RCD but there is no need for a live Zs reading.  Zs in the EICR form would be obtained by the measured R1+R2 test which remember can be used to verify correct polarity as well as R2 (cpc's resistance) and added to the single live test at the CU to measure Ze, and so the columns Zs, R1+R2, polarity verification and R2 columns (4 in total) can be filled in.  But if the RCBO has been installed for the reason of providing "additional protection" such as where PVC sheathed cables are buried at a depth less than 50mm from the plastered surface of a wall.

If you find a higher reading for an existing installation's Zs than permitted then the Regs say you can either rewire that particular final circuit using a heavier cable, add local equipotential bonding or fit an RCD.

pod to an existing CB.

It's within existing installations we are sent to test that we can get high readings, not jobs we install.  A few of the guys have implied that they never come across this.

Now, Reg 415.1.1 and 415.1.2 are under the sub-heading of "Additional Protection: RCDs" which I have covered above.

 

[says-les]

In answer to sidewinder re the L-N reference, remember that Zs has nout to do with SC protection, but rather only to do with EF protection.

 

[says-les]

In answer to Technician (Senior Member) it is likely that Ipssc is less than I pfc due to parallel path effect, i.e. the resistance of R2 could be less than R1 and hence less than Rn! Ipssc flows through the R1+Rn loop not through the R1+R2 loop! No?

 

[says-les]

In answer to the Technician's post re Reg 415.1.2 this Reg is under the sub-heading of Additional Protection: RCD. I have covered that in 3 posts back. No one is suggesting that an RCD replaces a CB, in fact if it is in the form of an RCBO it could well be that an RCD pod has been added to the MCB, in which case it's impossible to have one without the other. Yes?

 

[Sidewinder]

says-les,

I am fully aware of the points you raise, I'm not following your train of thought though.

In a basic installation such as a domestic dwelling, then an RCD will almost certainly be required these days.

It seems that you are not advocating the removal of the over current protection, OK.

However, fitting of an RCD is additional protection IMHO, it is not a replacement for basic circuit design & construction in compliance with BS7671.

Ipscc will only ever be less than Ipefc on a single phase installation though remember.

Yes it is "possible" to have the L-E loop less than the L-N loop, I've seen this many times on a site that I do a LOT of work on as there is a LOT of steel around and in the ground, thus hundreds of parallel earth paths back to the local Tx, which is within the steel frame of the building.

On this site it is not possible to measure Ze even with a black building, it is only ever possible to measure Zsdb at the origin, & the DB is not much like a DB either!

RCD's are additional protection, Zs should be within that required for ADS in BS7671 for the "mcb" for competent design.

IF this cannot be achieved by conventional means then the last resort would be to move to RCD protection due to their inherent lack of reliability.

It's just like this damn fool idea that seems to be going around these days, that 2 core SWA is no longer allowed i.e. no SWA as a cpc, or that a separate cpc must be run with an SWA.

BTW says-les, this last comment is not aimed at you!

 

[says-les]

Thanks to Sidewinder. I follow all your thoughts but in my case I am dealing with a church that has originally had one dist board of fuses and was upgraded ten years ago to C-Type CBs. The dist board is TP&N and two of the socket circuits were over the Zs limit. None of the cabling is below plaster. When RCD pods were added to two such circuits they showed up neutral to earth shorts by not setting, faults that could easily been present from the 20 year old build. 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. It is about 40 metres to the first/last socket-outlets on the ring in some cases. When tested, after the faulty leg of each ring was replaced and proper ring tests applied, it was found that the Zs for the C-type CB's was above that required by Regs Table 41.3 after the 0.8 factor is applied. The fitting of the RCD pods to the C-type CB's was therefore a remedy for Fault protection inadequacy of the existing CBs. This means that we can forget the Min Zs required per Table 41.3. The so formed RCBOs must still pass the RCD tests and Zs is sort of made redundant in this case.

There is no need therefore to test live for the measurement of Zs but by doing the R1+R2 reading and adding to the measured Ze reading we can complete all aspects of the EIC Schedule of Test Results.

From what I can gather some of the guys in the forum seem to be thinking that RCD means one per half of a CU but in this case there is one RCBO for each Power circuit.