High Zs values (Just)

[Deleted member 9648]

Given that Amd3 reduced max Zs values slightly what is everyone coding if a measured reading falls between the pre Amd3 value and the post Amd3 value. Failure to meet disconnection times would normally be a code 2, but a code 3 seems appropriate here......maybe a note to explain? On E-certs it will show a measured reading exceeding the maximum value given in the schedule.
Just seems silly that not long back a reading met disconnection times and now it doesn't, nothing has actually changed.

 

[telectrix]

hmm. if you consider the load on the circuit as opposed to the In of the OCPD, then you can assume that the cable may never reach 70 deg.C and so no need to apply the 0.8 temp. correction. tin hat glued in place.

 

[Ian1981]

I’m not sure we have any wiggle room to be honest when using max zs values.
It is what it is for the British standard to what it complies to or what the manufacturers state if asked.
I would apply a C2 if you can’t meet the stated disconnection times stated in bs7671 ie 0.4 or 5 seconds for TN arrangements.

 

[Flanders]

This is what frustrates me about coding .It was fine last year no code but this year its dangerous ( according the regs), No saying that I would give it a code 2 as it was poorly designed in the first place if it was so close the the maximum permitted value, My reasoning for this is we are finding things all the time that were fine years ago but now with more knowledge are finding out things are not so safe ( asbestos for example) .I have always given code 3 for no RCD protection for cables burred in wall , but the other day a contractor got an electric shock because someone had screwed up a metal display unit and one if the screws went straight into the live conductor of the twin and earth cable making the unit live (No rcd protection ) so now i`m thinking should I be giving cables burred in walls less that 50mm a code 2

 

[Wilko]

Perhaps it's low current (no trip) earth loop measurement inaccuracy? And you could mark it for Further Investigation?

 

[westward10]

You have to draw the line, either dither about thinking it was okay x amount of years ago and flip a coin or follow the new values blindly, regardless. I am with #3 on this.

 

[Deleted member 9648]

I’m not sure we have any wiggle room to be honest when using max zs values.
It is what it is for the British standard to what it complies to or what the manufacturers state if asked.
I would apply a C2 if you can’t meet the stated disconnection times stated in bs7671 ie 0.4 or 5 seconds for TN arrangements.

Asking because this exact scenario has cropped up today, going with a code 3 and a note because I cant bring myself to state an installation that was compliant 3 years ago is now potentially dangerous. A further complication is that on E-certs the measured value will exceed the stated max value hence the need for a note I feel. Where this crops up in future I'll need to refer to pre Amd3 regs as well as the new if I take the code3 approach.
It's a bloody minefield.

 

[Deleted member 9648]

Perhaps it's low current (no trip) earth loop measurement inaccuracy? And you could mark it for Further Investigation?

No mate it's a good reading (high current) and pretty well matches the value on the previous EICR.

 

[westward10]

Have you measured Uo and calculated what the max Zs would be although voltages can change.

 

[Ian1981]

T

Asking because this exact scenario has cropped up today, going with a code 3 and a note because I cant bring myself to state an installation that was compliant 3 years ago is now potentially dangerous. A further complication is that on E-certs the measured value will exceed the stated max value hence the need for a note I feel. Where this crops up in future I'll need to refer to pre Amd3 regs as well as the new if I take the code3 approach.
It's a bloody minefield.

Thats your call but the cmin factor was added in to provide compliance with the scenario of a drop in supply voltage.
I hear you about changes that now make installations potentially dangerous, just look at the wiring in escape routes headache and now the 18th will introduce metal supports against premature collapse to the entire installation

 

[plugsandsparks]

Hasn't happened to me yet but the EICR i am currently doing (industrial) has always been at the ragged edge as i cannot convince the local electricians to go up a size in cable for long runs, however they produce their own LV on-site with auto tap changes so i wont be coding anything just because of the AMD3 changes to a voltage i have yet to actual see (ever , anywhere) concrete hat on, lol

 

[Nigel]

Picking and choosing which Reg to follow is a slipery slope in my opinion.

 

[Ian1981]

I think the lowest voltage I’ve ever measured is about 220 volts

 

[Nigel]

This issue is solved relatively easily and cheaply by installing an RCD on the circuit in wuestion.

Code it appropriatly.

 

[Deleted account]

Have you measured Uo and calculated what the max Zs would be although voltages can change.

Shirley the multi function tester takes into consideration the measured voltage when giving the Zs value.

 

[telectrix]

Shirley the multi function tester takes into consideration the measured voltage when giving the Zs value.

maybe so, but all is eclipsed by the moon.

 

[Deleted account]

maybe so, but all is eclipsed by the moon.

?

 

[Deleted member 9648]

This issue is solved relatively easily and cheaply by installing an RCD on the circuit in wuestion.

Code it appropriatly.

Don't get where you are coming from Essex....why does the cost of rectification affect the code applied?

 

[Deleted member 9648]

maybe so, but all is eclipsed by the moon.

You been on the beer Tel?

 

[Toneyz]

You been on the beer Tel?

If not he'll be seeing the stars soon.

 

[Nigel]

Don't get where you are coming from Essex....why does the cost of rectification affect the code applied?

For me it does not. I would code it as per 7671. But the op is concerned and if the remedial works are relatively easy then I cannot see why he is even considering fudging it.

 

[Deleted member 9648]

For me it does not. I would code it as per 7671. But the op is concerned and if the remedial works are relatively easy then I cannot see why he is even considering fudging it.

You've lost me mate, does that mean you'd code 2 or 3? Absolutely everything I code on an EICR is 'as per 7671'......don't you?

 

[Richard Burns]

I have not actually come across this yet, though it has been close at times. If this were found then I would expect that some careful retermination of connections would resolve the problem, you are talking a fraction of an ohm here.
Unfortunately with circuit breakers if the resistance level were exceeded then the disconnection time could jump from practically instantaneous up to 13s (if the circumstances were just right, the type B CB is on the upper limit of design(5In) and the supply voltage is down to 218V) this tends to mean that non compliance could be significantly dangerous, though as you say it was deemed safe three years ago, only the definition of dangerous has changed.
If it were a fuse then a faction of an ohm increase would not significantly increase the danger as the time would be 0.401s instead of 0.4s and this might be suitable to be coded as improvement recommended. I think (silly as it sounds) if it is a circuit breaker then a potentially dangerous situation by the new definition of dangerous is now in place.

 

[Nigel]

You've lost me mate, does that mean you'd code 2 or 3? Absolutely everything I code on an EICR is 'as per 7671'......don't you?

I would code it a 2.

 

[Toneyz]

If you take a 32A type B circuit breaker the max Zs under 15th was 1.50 under the 16th the voltage was based upon 230V and was 1.44 now with Cmin reduced further to 1.37 the reduction being 0.13 ohms you can see how this has come about.

 

[Deleted member 9648]

Thanks for the replies everyone.
I'm sticking to a code 3 with an explanatory note but will discuss the issue with my NICEIC inspector on his next visit and go with his interpretation. I understand the reasoning behind a code 2, but I think it would be very hard to explain to a client that his fully compliant installation wired 3 years ago is now deemed potentially dangerous.

 

[Nigel]

Thanks for the replies everyone.
I'm sticking to a code 3 with an explanatory note but will discuss the issue with my NICEIC inspector on his next visit and go with his interpretation. I understand the reasoning behind a code 2, but I think it would be very hard to explain to a client that his fully compliant installation wired 3 years ago is now deemed potentially dangerous.

It will be in harder to explain why you think you can just ignore parts of BS7671.

 

[Deleted account]

Essex,
I don't believe wire puller has ignored bs7671.

He has made an informed decision and given a code.

I actually agree with you and would give the C2, but I agree it's a bit unfair considering the changes in the book.

The individual spark doing the EICR must code appropriately to how they personally view the issues with suitable explanation.

 

[Nigel]

Essex,
I don't believe wire puller has ignored bs7671.

He has made an informed decision and given a code.

I actually agree with you and would give the C2, but I agree it's a bit unfair considering the changes in the book.

The individual spark doing the EICR must code appropriately to how they personally view the issues with suitable explanation.

Maybe he will write down that it is only just a little bit dangerous.

 

[static zap]

EU & harmonization , a simple idea goes and gets all complicated .
( have DNOs got 230V for a rainy day ? )

 

[Intoelectrics]

I have not actually come across this yet, though it has been close at times. If this were found then I would expect that some careful retermination of connections would resolve the problem, you are talking a fraction of an ohm here.
Unfortunately with circuit breakers if the resistance level were exceeded then the disconnection time could jump from practically instantaneous up to 13s (if the circumstances were just right, the type B CB is on the upper limit of design(5In) and the supply voltage is down to 218V) this tends to mean that non compliance could be significantly dangerous, though as you say it was deemed safe three years ago, only the definition of dangerous has changed.
If it were a fuse then a faction of an ohm increase would not significantly increase the danger as the time would be 0.401s instead of 0.4s and this might be suitable to be coded as improvement recommended. I think (silly as it sounds) if it is a circuit breaker then a potentially dangerous situation by the new definition of dangerous is now in place.

As you stated if the readings are very close to the stipulated maximum values then it could be just a matter of checking the terminations at accessories. I have had this situation many times especially on RFCs, which can flag up when you do end to end measurements and you get high or inconsistent readings. Many times it has turned out to be a loose connection or a poorly terminated conductor.
I think "wirepuller" is in a tricky spot as he has to explain to the client that prior to AMD3 there was no problem but now it doesn't comply. Then explaining to the client that the installation is potentially dangerous will be awkward also because they will likely not understand how it can be completely safe a few years back but now dangerous, yet there have been no alterations or deterioration?
Electricians these days have to be highly skilled consultants, negotiators, teachers and salespersons.

 

[Intoelectrics]

Duplicate post

 

[telectrix]

so, this Cmin. causes problems. samewith the temp.0.8. never ever seen a supply voltage as low as 218V, never seen a cable reach 70 deg.Cunless overloaded.just justification for the IET trough feeders high earnings.

 

[Ian1981]

so, this Cmin. causes problems. samewith the temp.0.8. never ever seen a supply voltage as low as 218V, never seen a cable reach 70 deg.Cunless overloaded.just justification for the IET trough feeders high earnings.

I think the point is that it could happen.
Cables designed for maximum operating temperature of 70 degrees can be allowed to reach this temperature and thus allowances must be made for this scenario.
The voltage matter......same as you never recorded at voltage so low before but again it’s erring on the side of caution and above all else, safety

 

[Deleted member 9648]

It will be in harder to explain why you think you can just ignore parts of BS7671.

Maybe he will write down that it is only just a little bit dangerous.

Not all non-compliances with current 7671 will attract a code 2. For example the recommended code for a cable buried at <50mm deep without RCD protection and compliant with recent previous editions of 7671 is a code 3.
That's not 'ignoring' BS7671, it is recognising that while it does not comply with the latest edition, it complied with a recent but previous edition and was deemed safe then so cannot suddenly be dangerous.
If your logic applied then any code 3 would be 'ignoring' Bs7671, there would only be a code 1 or 2 permitted.

 

[Nigel]

Not all non-compliances with current 7671 will attract a code 2. For example the recommended code for a cable buried at <50mm deep without RCD protection and compliant with recent previous editions of 7671 is a code 3.
That's not 'ignoring' BS7671, it is recognising that while it does not comply with the latest edition, it complied with a recent but previous edition and was deemed safe then so cannot suddenly be dangerous.
If your logic applied then any code 3 would be 'ignoring' Bs7671, there would only be a code 1 or 2 permitted.

You understanding of a C3 is not correct I am afraid.

 

[Ian1981]

Thing is we are required to inspect and test to the current edition of the wiring regulations and for us to achieve ADS as well as the protective bonding to be in place, then the formula
Zs x Ia must be equal to or less than 230v x cmin (0.95).
If the circuit(s) can’t fulfill that formula then I can only see a C2 option as ADS can not be guaranteed at all times, at least not according to bs7671.

 

[Deleted member 9648]

You understanding of a C3 is not correct I am afraid.

Kindly explain it to me then.

 

[davesparks]

Not all non-compliances with current 7671 will attract a code 2. For example the recommended code for a cable buried at <50mm deep without RCD protection and compliant with recent previous editions of 7671 is a code 3.

Compliance with previous editions, recent or not, does not affect the code given.
The code given is in accordance with the current edition.

 

[davesparks]

so, this Cmin. causes problems. samewith the temp.0.8. never ever seen a supply voltage as low as 218V, never seen a cable reach 70 deg.Cunless overloaded.just justification for the IET trough feeders high earnings.

Out of interest how often have you measured the temperature of the conductors within a cable in service?

 

[telectrix]

not often. last time was a 16mm T/E running @ 100A. it reached 40 deg.C. after 30 minutes when the 63A MCB tripped. that's the sheath temp, conductors probably a bit warmer.

 

[davesparks]

not often. last time was a 16mm T/E running @ 100A. it reached 40 deg.C. after 30 minutes when the 63A MCB tripped. that's the sheath temp, conductors probably a bit warmer.

Exactly, not often, hardly a good sample size to make a conclusion about every cable in every installation in the country.

 

[Intoelectrics]

Exactly, not often, hardly a good sample size to make a conclusion about every cable in every installation in the country.

I have often come across cables that are worryingly hot during normal service. Though I have not actually measured the temperature of many some have been too hot to touch. I'd say (depending on the install) temperature is a very serious factor to consider when doing an install.

 

[telectrix]

Exactly, not often, hardly a good sample size to make a conclusion about every cable in every installation in the country.

i agree, mainly in industrial situations, but generally, cables are underrated by the BS7671 to err on the side of safety, espin the domestic sector where cables are rarely pushed anywhere near their rated capability.

 

[Deleted member 9648]

Not all non-compliances with current 7671 will attract a code 2. For example the recommended code for a cable buried at <50mm deep without RCD protection and compliant with recent previous editions of 7671 is a code 3.

Compliance with previous editions, recent or not, does not affect the code given.
The code given is in accordance with the current edition.

The post you have quoted clearly states that codes are allocated according to the current edition of 7671, I have not suggested otherwise.
But my interpretation of a code 3 issue is one that does not comply with current 7671 but in the opinion of the inspector does not compromise the overall safety of the installation. I'm still waiting for Essex to explain his interpretation to me.
We are also advised that when carrying out an EICR consideration must be given to the fact that an installation complied with Bs7671 at the time it was installed. For example for an installation wired in the last (say) 20 yrs, lack of RCD protection to socket outlets not likely to supply equipment outdoors would attract a code 3, not a code 2.
I am still not persuaded that a zs reading which was compliant 3 years ago is now potentially dangerous.

 

[plugsandsparks]

It clearly isn't dangerous and i am at a loss understanding the need to introduce it, there is obviously a downside as cabling and distribution costs will obviously rise, breakers will be much more on the limit of the load and may cause nuisance tripping, so balance this against an install that might only have 220V and simultaneously have a short cct fault. Since i have never measured this voltage on any of the commercial and industrial installation for the last 10 years, i am at a loss to why this obviously costly reg was introduced.

 

[Nigel]

The post you have quoted clearly states that codes are allocated according to the current edition of 7671, I have not suggested otherwise.
But my interpretation of a code 3 issue is one that does not comply with current 7671 but in the opinion of the inspector does not compromise the overall safety of the installation. I'm still waiting for Essex to explain his interpretation to me.
We are also advised that when carrying out an EICR consideration must be given to the fact that an installation complied with Bs7671 at the time it was installed. For example for an installation wired in the last (say) 20 yrs, lack of RCD protection to socket outlets not likely to supply equipment outdoors would attract a code 3, not a code 2.
I am still not persuaded that a zs reading which was compliant 3 years ago is now potentially dangerous.

It is really very simple. A c3 is an issue that is not dangerous if left but could be improved using the current Edition of 7671 as a guide.

So anything that is not done to 7671 but is not classed a C1 or a C2 would be a C3.

 

[Deleted member 9648]

Er...that explanation looks pretty much the same as mine

 

[Nigel]

Er...that explanation looks pretty much the same as mine

It read as if you were saying if it meets a previuos version of 7671 then you would no code it.

 

[Deleted member 9648]

It read as if you were saying if it meets a previuos version of 7671 then you would no code it.

Ok, wasnt what I meant,