Will Zs measurements be redundant under the 17th Edition?

[PC Electrics]

Here's a clue:

RCD = supplementary protection......

 

[Shakey]

But if the circuit is on an RCD anyway, the Zs measurement becomes irrelevant

The way I see it is
R1 + R2 + Ze = Zs
The R1 may have a very high resistance perhaps through a loose connection or faulty accessory that may cause a fire if the overheating is allowed to continue un noticed
The presence of an rcd will not help if the zs of the circuit is not checked and just assumed to be under the maximum for the rcd
If the standard test is carried out this would be noticed
So sorry to take the opposing view but I still think the test is relevant

Des, I didnt say dont check the circuit

You would still do R1+R2 (and find the loose connection), and still do Zs calculated or measured (EAWR's permitting)

What I am saying is lets say the Zs is 1.6 ohms for a 32A 60898, what are you going to do? You are going to put an RCD on

But the RCD is (probably) gonna be there anyway, so I am saying, what differnece does it make if you measure 1.2 or 1.6 or 2.4 or 3.6 or 4.8 .........all the way up to 1667 ohms

I am not saying dont measure (or calculate)

I am saying that the actual reading will be largely irreleveant

Obviously if you are working on a PME system and come out with 47ohms Zs for your ring, you would investigate the problem (although you should already have done that with you ring continuity checks)

But barring any wiring problems (ie any high reading is down to Ze) the Zs will have little meaning

And lets be honest -you dont even have to measure it anyway!!!!

Its good to talk chaps.....

 

[mijo]

My understanding is that an RCD will not cope with large earth faults currents. For example if the earth loop impedance (ELZ) was say 0.5 ohms you could have an earth fault current of 460 amps.
It is difficult getting manufacturers data on RCDs especially on this point ( max breaking current ) but I have seen a Hager spec for an RCD which quoted max of 200 Amps If you take an inline or plugtop RCD to bits it is fairly obvious that they will not cope with large earth fault currents and many installation type RCBO's have their RCD operation mechanically linked to the MCB the RCD will trip first but arc under heavy earth fault currents but it's mechanical trip actuates the MCB whose contacts can handle the heavy earth fault current without arcing over and break the fault current obviously the mechanical tripping of the RCD and it's arcing under large earth fault currents and the RCD's mechanical tripping of the MCB takes the breaking time outside that permitted for an RCD thus the ELZ for the MCB is required by the regs.
It should also be noted that the MCB deals with short circuit current I know this is Neutral/Live loop impedance which often in domestic installations comes under PFC (prospective fault current) but PFC must be checked both for earth fault current (earth loop impedance) and short circuit fault current (neutral-live loop impedance.

 

[jimmus]

Think the confusion rises when we talk of RCD or RCBO

Having this out with WYLEX at the moment.

17th ed- 411.4.9 Where RCD used to satisfy reg 411.3.2.2 (max disconnection times) max Zs value from table 41.5 can be used. However table 41.3 states "overcurrent characteristics of RCBO's to 61009-1"as its the overcurrent part of the RCBO that is being looked at not the earth fault part.

I am trying to get type B RCBO's to comply with 41.3 but their rep states they dont need to manufacture type B as type C comply with Table 41.5 (1667 ohms Zs) so thats what they make.

So if you have a 32A ring final circuit with Zs of 0.9ohms it is ok on type B (1.44 max) but not on type C (0.72).

I may be wrong and only the "magic 1667" figure is the only one that matters, but even if this is the case table 41.5 only applies to circuits not exceeding 32 amps so if you have for example a 9.8KW shower wired in 10mm on a 40 amp RCBO you still have to comply with 41.3 as 1667 is not applicable.

So Zs becomes relevant again.

 

[Shakey]

Ok chaps

just watched a joint NICEIC/Megger DVD on intitial verification

They state on there that where circuits are RCD protected, the Zs reading need only be taken with a low resolution meter capable of measuring 1667 ohms

I also have in front of me a copy of "Guide to the Wiring Regulations BS7671:2008"
published by no less than the ECA

Page 63 has the following quote under the heading "Irrelevant ELI specification"

"A common source of misunderstanding is that of either specifying or measuring values of ELI where the circuit also has an RCD fitted. ELI measurement under these circumastances is a futile exercise. The circuit will have been checked for continuity, and this is what is needed, together with, of course, functional checks of the RCD...........and for clarification, circuits with RCBO's do not need to meet the specified ELI values"

So back to the original post - will Zs values be irrelevant under the 17th edition? (for RCD protected circuits)


Well I think they will
NICEIC think they will
Megger think they will
The ECA think they will

and for information, the IET often adopt the ECA's reccomendations

*shakey takes a bow*

Hate to say i am right chaps, but, ........erm........point proved i think

 

[mijo]

Think the only way out of this information impasse is to approach manufacturers and ask for the max fault current that their RCD's and RCCBO's will withstand WITHIN STATED RCD OPERATION TIMES i.e. mS.
I suspect in many cases these withstand currents will be less than the earth fault loop impedance will let through in the event of an earth fault .
Then one relies on the MCB part of the RCCBO or a Fuse to break the fault current at much longer operating times and ELZ becomes highly important.
It is not unknown for max withstand current for RCD to be say 200 amps and min operating current for MCB to be 300 amps (30 amp type C MCB) ELZ 0.64ohms is imperative.
Regards Mijo

 

[a.walk]

Ok chaps

just watched a joint NICEIC/Megger DVD on intitial verification

They state on there that where circuits are RCD protected, the Zs reading need only be taken with a low resolution meter capable of measuring 1667 ohms

I also have in front of me a copy of "Guide to the Wiring Regulations BS7671:2008"
published by no less than the ECA

Page 63 has the following quote under the heading "Irrelevant ELI specification"

"A common source of misunderstanding is that of either specifying or measuring values of ELI where the circuit also has an RCD fitted. ELI measurement under these circumastances is a futile exercise. The circuit will have been checked for continuity, and this is what is needed, together with, of course, functional checks of the RCD...........and for clarification, circuits with RCBO's do not need to meet the specified ELI values"

So back to the original post - will Zs values be irrelevant under the 17th edition? (for RCD protected circuits)


Well I think they will
NICEIC think they will
Megger think they will
The ECA think they will

and for information, the IET often adopt the ECA's reccomendations

*shakey takes a bow*

Hate to say i am right chaps, but, ........erm........point proved i think

Absolutely agree!! Continuity test will tell you all about the circuit (loose connections, circuit lenght etc). the rule is simple anyway if you have measured R1+R2 and also Ze you automatically have a Zs value without measuring.