Parallel paths headache.

[44ALLAN]

Hi guys,
I've spent the last few weeks reading through the site and have got to say I'm very impressed with the patience and knowledge you all have.
This topic has been discussed and i'm sorry for bringing it up again but I would still like your opinion on the way I test and record my Zs readings.
I'm in an Industrial environment and old school ( sparky for 38 year).
I'm the only spark on site and run my own maintenance and test scheme and unlike you guys don't get inspected by governing bodies etc.

So here lies my problem.
The factory where I work has 3 sets of metal housed overhead busbars fed with 70mm SWA cable.
These have fused tap off boxes which then drop down and feed wall Isolators which then feed D/Boards which then feed Machine Isolators which then feed machine transformers which then feed fixed machines.
All of these are metalclad and fed with Swa cable.
edit....I also always include a seperate earth in the feed cables and the supply is TNS 3p+N.

Now being 'old school' you can imagine I have bonded everything in site and then bonded wall isolators D/B's and busbars to the factory girders.
My transformer is on site and approx 20 metres away so you can imagine my Zs readings are absolutely perfect and well below the allowed figures............problem is this includes all parallel earth paths as it would be impossible for me to disconnect them all.

On my forms I record all the Zs readings from all the various points and put a side note saying these results include parallel earth paths.
I'm happy in my own mind that these extra parallel paths are permanent and will not be removed so treat them as part of the earthing system and therefore my Zs readings are acceptable albeit much lower than would be the case without the extra paths.
Am I right in this assumption or should I be doing something different?

I look forward to any input you may have it will be greatly appreciated.

Allan

 

[malcolmsanford]

Well Alan your Zs values if taken by measurement should include those parallel paths as that is how the installation will be when and if a fault occurs. I'm sorry but I'm not a calculation guy when it comes to Zs, I like the installation to be in the condition it is when in use.

I'm assume you are conducting PIRs on your installation? You do know of reg 622.2 concerning installations that are under effective maintenance system? By your post I would gather that you keep the installation pretty much under control, so you have no need to do PIR's to the same intensity as an installation that as no maintenance, or not even any PIRs if you wish.

 

[oxocube]

In theory your Zs should not include parralel paths but it is not practical or possible. Most modern buildings of any type contain structural steel, steel decking, steel floors, steel studwork, steel mechanical services and various earthing systems. And they are all permanently fixed together and comprise millions of parralel earth paths.

So theres nothing wrong with what you're doing.

 

[44ALLAN]

Malcolm,
I've basically set my own maintenance scheme up with all the relevant tests on a form of my own making.
Being on-site it is under my wing,so to speak,so I inspect and test etc to my timeframes but it is an ongoing situation.
In any one machine circuit for instance I will have 6 points where I have to take PEFC and PSCC readings so it gets pretty time consuming sometimes.
I also totally agree about the circuit being tested in the condition it will be in under fault conditions.

Oxo.
Thanks for the reply its good to have a bit re-assurance sometimes from fellow sparks and I'm glad that my method is acceptable, along with all the other tests of course

 

[Engineer54]

44allen,

I don't think i ever have seen an industrial installation of any size, that isn't full of intentional and unintentional parallel paths. I honestly can't see a problem with what you've described in your OP, your Zs is excellent, and to all intended purposes will remain so. There is nothing you can do differently, so don't worry about it, ...It sounds to me, that you have a more than decent installation there, and it's being maintained '' in house'', which is becoming more and more a rarity these days.

AS has already been said your doing nothing wrong, so don't worry about it and put your mind at rest ...lol!!

 

[Engineer54]

Malcolm,
In any one machine circuit for instance I will have 6 points where I have to take PEFC and PSCC readings so it gets pretty time consuming sometimes.
I also totally agree about the circuit being tested in the condition it will be in under fault conditions.

Silly Question, ...but WHY??

 

[44ALLAN]

Hi Eng 44,

PEFC and PSCC from all 'relevant points'.
1). Busbar tap off box fused at 100Amp BS88-2.
2).Isolator for D/Board fused at 80Amp BS88-2.
3).4 Way D/Board fused at 63Amp BS88-2.
4). Wall Isolater feeding machine transformer fused at 50Amp BS88-2.
5).Transformer Isolator 100Amp MCB
6).Machine Isolator 100Amp MCB 100mA RCD.

I treat each one as a seperate circuit as they all have circuit breakers with different breaking capacities and therefore I take readings at each point and record the highest value ( times 2) on my form.
The machine isolator for example has a capacity of 7.5kA.
Because my Zs is so low due to parallel paths my readings here are so good I was nearly having to upgrade the breaker.
I also find it saves on paperwork if I put all these test results on the same sheet and treat it as a full circuit instead of doing them all seperately.

Hope you understand my reasons and look forward to any suggestions you may have.

Allan

 

[Engineer54]

That's going way, way overboard, but hey, if your happy and have the time for making all those tests, ...I'm certainly not going to complain!! ...lol!! So how often do you undertake all your testing at the factory??

 

[MarkieSparkie]

Hi 44Allan, Your method and procedures seem spot-on to me. I also agree with the other guys that measured Zs of the in-service installation condition is the only meaningful way of assessment for the majority of commercial and virtually all industrial installations. The Zs calculation method in my opinion only yields meaningful results in the largely all-insulated world of domestic installations, still prefer to measure Zs in-service. It's always interesting watching and reading "all domestic" apprentices and newbies fantasy test results when they are confronted by an installation wired in singles in steel conduit, however they soon learn an important lesson regarding parallel paths.

 

[44ALLAN]

Engineer 54

I'm not saying I do all those tests everytime but they still have to be done ( and recorded) at ALL relevant points to prove the breaker is of the correct rating and the Zs is low enough to take out the breaker in the event of a fault at that point. If theres another way please let me know lol
My factory is not heavy industrial so I try within a 3 year period to have tested almost everything then start again !
I agree with the other guys that inspection and knowing your installation is the most important part then testing for conformity tags along with it.​

 

[SirKit Breaker]

Your testing regime in my humble opinion is spot on. You are the man on the ground and in the know about the type of use and abuse your kit gets, so you are best placed to decide what and when things get tested, inspected and so on. If you have an ongoing regime such as yours, then you should really be keeping a record of what you are doing, as this exempts you from having to comply with the Periodic Test and Inspection intervals. If you have no record, then this may lead to you having to do the whole lot again at one go to satisfy some pen pusher from an Insurance company or similar.

Cheers...........Howard

 

[44ALLAN]

Thanks for all the responses.

I was expecting a few 'negatives' but have realised most are of my opinion when 'limitations' come into play.

Everything 'reasonably practical' and all that but never become too complacent. That's why after all these years I asked the question.
Neva too old to learn and really glad to see over 100 viewings of the thread............hope it has helped others too.

Again

Many thanks to all that answered and viewed.

 

[Engineer54]

Engineer 54

I'm not saying I do all those tests everytime but they still have to be done ( and recorded) at ALL relevant points to prove the breaker is of the correct rating and the Zs is low enough to take out the breaker in the event of a fault at that point. If theres another way please let me know lol
My factory is not heavy industrial so I try within a 3 year period to have tested almost everything then start again !
I agree with the other guys that inspection and knowing your installation is the most important part then testing for conformity tags along with it.​

Sorry 44Allan, ....I was under the impression that you were doing all these tests in one go so to speak, not over a 3 year period.

All you really need to be testing is your Zs at these various points, to prove disconnection times. All those BS 88 fuses will have a breaking capacity of around 80kA and i don't think your going to see that sort of fault current on your installations. The MBCs (or are thy MCCBs) will have the breaking capacities marked on them as will any MCCB's. If MCCB''s then you will need to check with manufacturers data for min Zs requirement...

 

[44ALLAN]

Engineer 54.

To tell the truth the majority of my machines are only 3phase so a 4core cable would normally suffice(spare core being used as dedicated earth).Me, being a forward thinking chap though, I always install 5-core and terminate the neutral too in case we change machines in the future (or need a neutral for something else).
Like you say my fuses are normally 80kA but while I'm doing a Zs I always take another 20secs and do a PSCC and record the highest on my forms..........pedantic maybe but while I'm there it doesn't take long and it also helps me check against previous results as all the busbars are bolted together(and old) and any dodgy joint in the neutral loop would show up with a higher test from previous.
My machines have indeed Moulded circuit breakers and not Mcb's as I first stated.

It might just be me but my report sheets are turning into 'War and Peace' replicas

 

[mattmoo1974]

44ALLAN your rare! But the sounds of it you dont sit in the tea room all day..