IET logo
 
IET
Decrease font size
Increase font size
Topic Title: Zs folly
Topic Summary:
Created On: 03 February 2016 07:48 AM
Status: Read Only
Linear : Threading : Single : Branch
1 2 Next Last unread
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 03 February 2016 07:48 AM
User is offline View Users Profile Print this message



lyledunn

Posts: 1107
Joined: 13 August 2003

Came across two lads erecting tower scaffolding in the office reception area of a large warehouse facility. High ceiling with class two luminaries. I knew one of them so he stopped for a chat. Naturally I asked what they were doing. Getting Zs. Turns out they have done this every three years since their firm erected the installation twelve years ago! In a chirpy confident explanation the lad tells me that Zs is taken at the end of line light fitting ie the same fitting has now been tested at least four times. I imagine that this folly is not unusual. God help British industry!

-------------------------
Regards,

Lyle Dunn
 03 February 2016 08:33 AM
User is online View Users Profile Print this message



sparkingchip

Posts: 9974
Joined: 18 January 2003

They have to use a tower to access the fitting terminals as being a Class II fitting they cannot stand on the floor and use a wander lead on a pole to test any exposed metalwork.

 03 February 2016 08:43 AM
User is offline View Users Profile Print this message



mapj1

Posts: 9383
Joined: 22 July 2004

I think the point is that at that height it could be defective and even been class 1 and have failed and become live chassis, and left for several years, and no more dangerous than a bare overhead for a crane or similar.
By all means take the kit up to check it is earthed when they already have to go up because a bulb needs replacement or clean the ventilation or any other task, but to do it specially just because a calender says so, to some largely imagined schedule, is a senseless waste of money and effort that could be far better directed elsewhere.

I'm sure that the only reason that there are so many folk performing nugatory annual tests, is because the words triennial, quinquennial and decennial are just far too hard to spell... Pity really, as better spelling education would save a lot of effort.

-------------------------
regards Mike
 03 February 2016 09:25 AM
User is online View Users Profile Print this message



sparkingchip

Posts: 9974
Joined: 18 January 2003

The point is it is pointless.

Knowing Zs at a class ll light fitting is only really relevant if you are about to replace it with a fitting that requires an earth.

Often it is more of an issue checking the unused CPC has been made off tidily at the fitting if the manufacturer has not provided a parking terminal.

Andy
 03 February 2016 10:05 AM
User is online View Users Profile Print this message



Jaymack

Posts: 5361
Joined: 07 April 2004

Originally posted by: lyledunn
I imagine that this folly is not unusual.

PAT is another that is probably being unnecessarily carried out, at the previous regimented intervals ........... which Joe Public is ultimately paying for.

Regards
 03 February 2016 07:12 PM
User is offline View Users Profile Print this message



spinlondon

Posts: 5494
Joined: 10 December 2004

Some sites I have worked on, require new items to be PAT tested.
 03 February 2016 07:45 PM
User is offline View Users Profile Print this message



jravenscroft

Posts: 73
Joined: 07 November 2015

Isn't the point also that even if it's decided in this scenario that the circuit Zs should be recorded every three years regardless of the unusual amount of effort required, the testing procedure still misses the point of checking the circuit earth.

The procedure laid out clearly in guidance note 3 is to check the quality of earth throughout the circuit, and to record the highest result. Not to check only the furthest point because you happen to know its where you'll get this result.

I say this, and perhaps it's not what the OP was thinking at all (the fact that they are doing what they are doing is comment worthy enough!), because I've recently been involved with testing blocks of new flats as part of a brand new large scale development project. Testers before me had been taking their r1+r2 results from the lights they knew to be furthest from the consumer unit, due to what I believe to be a lack of understanding of why we carry out these tests in the first place. CPCs not connected at various switches along the circuit, for example, could result in class 1 fittings being installed under the presumption of the presence of earth where none has been verified.

Sounds to me that in OPs situation, a company is being paid not only to carry out an unnecessary test, but also under the belief that they are fulfilling a duty which they may not actually be, thanks to a lack of understanding.

Of course I could just be projecting my recent experiences unfairly,
 03 February 2016 09:42 PM
User is offline View Users Profile Print this message



MrP

Posts: 955
Joined: 24 March 2006

In deeded Zs folly
"Zs" testing confirms absolutely nothing. It's a British cottage industry procuring the sale of book, tables, instruments worthless training courses and fills the backs on inspection reports with irrelevant numbers

MrP
 03 February 2016 09:57 PM
User is offline View Users Profile Print this message



leckie

Posts: 4329
Joined: 21 November 2008

Hello Mr. P.

Are you back home from your international duties?

So do we not need to confirm the Zs level is low enough to achieve ADS?
 04 February 2016 08:14 AM
User is online View Users Profile Print this message



sparkingchip

Posts: 9974
Joined: 18 January 2003

If you are making a spot check on Zs then it has to be acceptable to do it at a light switch with both feet on the floor.

This needs careful consideration testing is dangerous.

Andy
 04 February 2016 10:51 AM
User is offline View Users Profile Print this message



Zs

Posts: 3814
Joined: 20 July 2006

Ooh,

This makes me nervous because instictively I am so not agreeing with most of you about this. I applaud the three yearly check...might be a bit OTT compared to 5 years in an office but warehouse lighting in particular is subject to extraordinary demands and stresses.

If you are testing for Zs from inside a fitting you are also carrying out a visual inspection of the inside of the fitting.

I'll have to come back and expand later because I have a submission to finish today but for now, surely it depends on how the Zs test is run by which I mean along which conductors and relative to the earthing method of the installation.

Loads else for later but I'll watch for a while.

JR I read yours a few times because I also carry out a great deal of testing of huge new apartment complexes. However, I carry out Inspection as well as testing. What method would you recommend? That's not a challenge btw it is a genuine question.

Zs
 04 February 2016 01:59 PM
User is online View Users Profile Print this message



sparkingchip

Posts: 9974
Joined: 18 January 2003

Okay. Let's assume instead of a class ll double insulated light fitting that does not require an earth connection for safety it is the last of a line of class l fittings with a metal enclosure fixed with metal fixings or hanging on chains from the purlin on clips in a steel framed building.

What will testing Zs at the terminals prove?

The CPC could have failed, but it is earthed anyway through the extraneous metalwork.

Erecting a tower to take a reading is not going to be productive in most cases.

Andy
 04 February 2016 04:30 PM
User is offline View Users Profile Print this message



Zs

Posts: 3814
Joined: 20 July 2006

Hmm, granted that one is a bit of a case Andy, sort of what I'd consider to be an exception but only because you've hung it from the metal work. We have to consider all of them though so I'll not encourage you to get involved in discussing bonding the coins in your pocket (remember that one?)

If the CPC has failed then the reading you get from your test, unless you're doing it down the neutral, will be quirky because you are an experienced electrician and even before you press the test button you know the ball-park that you're going to be in.

You may even be able to compare it with the test results from three years' ago.....

When you're testing a large building you get to notice when a reading is awry.

Zs
 04 February 2016 04:40 PM
User is offline View Users Profile Print this message



Zs

Posts: 3814
Joined: 20 July 2006

Ok, while my big relux calculation is chugging away on the other computer..

At the end of a row of class 2 fittings , probably inside it and nicely terminated, you will have a CPC which is protecting the cable supplying the fittings and protecting those who choose to get too close to it's cores at any time. The CPC will be connected through each fitting in the row and will not have been chopped off will it?

I don't like this testing at switches for a recordable result. I like them to be tested but the Zs, IMHO, does need to come from as close to the end of a set of lights, i.e from the longest run of cable in the circuit.

Your Zs test will prove that the cable is adequately protected and that the associated risks of not having a CPC in the cable have been checked for.

Yes, I open up class 2 fittings all the time. You'll not be surprised to know that they oft don't have the CPC connected through.


Zs
 04 February 2016 04:58 PM
User is offline View Users Profile Print this message



jravenscroft

Posts: 73
Joined: 07 November 2015

Originally posted by: Zs
JR I read yours a few times because I also carry out a great deal of testing of huge new apartment complexes. However, I carry out Inspection as well as testing. What method would you recommend? That's not a challenge btw it is a genuine question.


Hi Zs,

In this case where most rooms had a single pendant and the radial fed the switches before the lights, it was quite practical for me to take an r1+r2 at each pendant in each room. At the same time I could verify that the switch was switching the line conductor, and could visually inspect the fitting and connections. I rarely take an actual Zs of a final circuit unless some other circumstance makes it appropriate.

I came across several faults that others would have missed my taking only one reading, such as discovering the occasional un-terminated cpc in a switch that I wouldn't otherwise have gone to the trouble of opening. It also helped to show me that although these finished flats were mostly identical, the end of line was not as easy to predict as you might think. It's entirely possible that someone could guess wrong using the single test method. Obviously not applicable to OPs scenario.

I've done similar in larger scale warehouse type installations, and I try and take the readings at different points each visit so that over an agreed time span each point has been checked. I wouldn't go to such extreme lengths as hiring a tower scaffold solely for the purpose of one test in a spot where harm was unlikely to occur in the event of a fault unless the client requested, but complacency is a sneaky beast, and I think it's best to verify rather than assume where practical.
 04 February 2016 06:18 PM
User is offline View Users Profile Print this message



Zs

Posts: 3814
Joined: 20 July 2006

Thank you for that reply, I am encouraged to read that.

However, I do use a Z test. Two lead, L-E usually. H&S would be after me for too much wander lead behaviour and I prefer not to dismantle a DB. It gets discussed prior to the event though.

I find that I see what I describe as signature defects in these large places and the same defect will be repeated all over in one block and then a different one all over the next block. Chopping off the CPC in a switch is pretty common.

Do you photograph them all?

I flagged something on Zs on the most recent one where there were two sockets in corridors, about 20m apart. I was looking for the end of the radial and noticed that the difference in the Zs's was really big. It was the same on each floor so I guess that the cable takes a convoluted route but I did flag that to be understood. All within limits but the sums don't work on it.

Every socket gets a Z taken because it is so easy to do it but I will put a limitation on the number of light fittings opened.

I'm sure quite a bit gets missed but getting to the end of a circuit is probably going to indicate an issue with further investigation required. Well, that's what I think.

I'm watching with interest and off to hide behind the sofa.

Zs
 04 February 2016 07:07 PM
User is offline View Users Profile Print this message



mikejumper

Posts: 2418
Joined: 14 December 2006

Originally posted by: lyledunn
I imagine that this folly is not unusual. God help British industry!

I'd say it's not unusual across all occupations.
 04 February 2016 07:23 PM
User is offline View Users Profile Print this message



spinlondon

Posts: 5494
Joined: 10 December 2004

Is it just me, or do all the industrial class II fittings that I see, have an earth terminal to earth the gear tray?
 04 February 2016 08:56 PM
User is offline View Users Profile Print this message



mikejumper

Posts: 2418
Joined: 14 December 2006

I suppose we need a proper definition of a class 2 fitting then.
 04 February 2016 10:33 PM
User is offline View Users Profile Print this message



mapj1

Posts: 9383
Joined: 22 July 2004

A Zs test at some point after installation and after any major surgery to the wiring or a change of fitting, is one thing, and very sensible.
Mindlessly taking it all to bits to re-measure it every year is not.
In the case of a fitting with nothing to be earthed within it, then maybe we need to be clear what do we think the CPC is protecting, and indeed if we really need it at all, for a reason other than 'the regs require it' ?
If a new metal bodied fitting is being installed, then that is the time to check, and not to do so then would be remiss..
If a fitting is earthed by the intended CPC or by another route, is that going to stop the ADS in the event of a fault ?
If not, is that a less serious fault than if so ?.
If you think there is a problem of large currents on unintended routes, then perhaps you should consider to also be doing Zs tests with high current to, as you could just as well have a legitimate CPC hanging by a thread.

There are many things to consider when deciding if a test is worthwhile.

-------------------------
regards Mike
IET » Wiring and the regulations » Zs folly

1 2 Next Last unread
Topic Tools Topic Tools
Statistics

New here?


See Also:



FuseTalk Standard Edition v3.2 - © 1999-2017 FuseTalk Inc. All rights reserved.

..