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Discuss testing for Ze and PFC. in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

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Ross Trician

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Evening all,

Can you give me some advice. I have been to do some testing today at a premises that is on a farm which has a 3phase +Neutral incoming supply. as a TT system. there are no signs of any earthing arangments what so ever.
the part of the system I have been to look at is fed via on of the phases plus the neutral thru a 63amp fused isolator. out of the isolator there are 2 feeds each going to its own CU in diferent areas again there are no signs of any earthing at either of the CU's.

question 1, because there is no earthing arangment I cannot take a Ze reading, ( obviously as theres no earth). would this also not mean then that the Zs reading for each cct is unobtainable too?

question 2, whilst testing for the pfc between phaseL and phaseN at one of the CU's at the main isolator with it in the of position I get a reading of 70A, would this be a true reading or is it likely to be being affected by the suplly to the other CU? would I need to disconect one of the CU's at the fused isolator to make them "individual" ccts?

Question 3, to rectify the fault of the earthing, I would propose to put an earth spike/electrode in the cct, 1 for each CU and then change the CU for the correct type for a TT system, (it is not right at the minute) what are your thoughts on this as opposed to using on spike for the "whole system at the incoming source. trouble is I dont really want to get involved with the "rest of the system as it has dis boards and feeds going all over the farm to various units etc.

thanks for any help, appreciate it. Ross
 
M

MacSparky

  • Thread Starter Thread Starter
  • #2
Wow!! cant see this thread going long without some good input, i will be watching this myself ross, an interesting situation you got yourself there mate:)
 
R

Reg Man

  • Thread Starter Thread Starter
  • #3
Question 1 - Yes you can't get a reading of Ze at the supply. Yes & No is the answer for the Zs of each circuit as the earth in the T+E (if it has earth) may be connected to frame of farm buildings, old lead water pipes etc etc. It could get a earth of sorts that way. Have you tried a plug in tester just to see if it reads an earth?

Question 2 - Turn both isolators off the CUs and measure the PFC.

Question 3 - I would phone electricity board and see if PME is available first. You may be surprised even if the place is in the sticks. Depends how good supply transformer is though.
 
S

Shakey

  • Thread Starter Thread Starter
  • #4
Evening all,

Can you give me some advice. I have been to do some testing today at a premises that is on a farm which has a 3phase +Neutral incoming supply. as a TT system. there are no signs of any earthing arangments what so ever.
the part of the system I have been to look at is fed via on of the phases plus the neutral thru a 63amp fused isolator. out of the isolator there are 2 feeds each going to its own CU in diferent areas again there are no signs of any earthing at either of the CU's.

question 1, because there is no earthing arangment I cannot take a Ze reading, ( obviously as theres no earth). would this also not mean then that the Zs reading for each cct is unobtainable too?

question 2, whilst testing for the pfc between phaseL and phaseN at one of the CU's at the main isolator with it in the of position I get a reading of 70A, would this be a true reading or is it likely to be being affected by the suplly to the other CU? would I need to disconect one of the CU's at the fused isolator to make them "individual" ccts?

Question 3, to rectify the fault of the earthing, I would propose to put an earth spike/electrode in the cct, 1 for each CU and then change the CU for the correct type for a TT system, (it is not right at the minute) what are your thoughts on this as opposed to using on spike for the "whole system at the incoming source. trouble is I dont really want to get involved with the "rest of the system as it has dis boards and feeds going all over the farm to various units etc.

thanks for any help, appreciate it. Ross
Q1 - as said, any Ze (or Zs) would be due to extraneous conductive paths

Q2, that reading would be the tru reading and the other CU could not effect it

remember that because it is thrre phase, the reading should be doubled anyway, but is still incredibly low

this represents a line to neutral Z of 3.2 ohms! Is it the same for each line to N?

Consider if a circuit was on a 32A Type B 60898, that takes between 90-150A amps fault current for instantenous trip, abd thats before you add any R1+R2 values, which would of course increase Zs and lower you fault current

potentially, you MCB's (or other CPD's) might never disconnect under fasult conditions, or at least would take a long time, wwhilst you can provide addtional protection with an RCD, as you know, that will not help you under a short circuit fault situation

Q3, means of earthing should be at the origin. each disboard feed is then a submain, with its own earth cable going back to the MET, to which your electrode earth will be connected

Are you working to 16th or 17th

If its the 17th then front end 100mA s types have pretty much gone, because most everything will be on a 30mA

hope this helps
 
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R

Ross Trician

  • Thread Starter Thread Starter
  • #5
evening chaps,
just to fill you in on a few more bits and to make things a bit clearer.

The incoming supply mis 3 phase which then uses the individual phases to feed various isolators and dis boards etc.

these are somer of the test results I did get.

ring was, L-L, 0.69 ohms. N-N was 0.66 ohms, cpc-cpc was 0.98 ohm.
L-N figure 8 1.34 ohm.
L-E fihure 8 was 1.56 ohm.
R1+R2 was 0.45ohms.
Ring Zs was 3.64ohms

Shakey, you say that the pfc is the right reading but needs to be doubled because it is 3 phase, can you tell me what difference that makes at the C/U as it is only fed with a live and neutral.

also can some of you who are more knowledgeable than me confirm I am interpreting this right,
in GN3 it states the max Zs for a type 1, 16 Amp, bs3871 CB is 3 ohms?
and for 32Amp CB it is 1.5ohms.
therefore the ZS readings I have are too high and presumably due to there being no earthing arangments???

also you say about putting a spike in for the whole system, would this then not make me "responsible" for the whole system?
my thoughts were to simply provide the earthing for the instalation I am working on and if they wish to "put right" the rest of the system then that would be a seperate issue.

I am initially working to the 16th, going on the 17th update course in couple of weeks.
could I put an rcd unit inline after the isolator to cover the whole system rather than changing both boards, advice on you ideas of instalation experience appreciated.

thanks Ross
 
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W

wayne

  • Thread Starter Thread Starter
  • #6
you only double the psc when testing at a 3ph point ie at the origin on a 3ph or at the db on 3ph sorry if im telling you what you already know
 
R

Ross Trician

  • Thread Starter Thread Starter
  • #7
Wayne, appreciate any coments. didnt know about the 3 phase "doubling" for the psc. is that if testing between phases or if tested live to neutral.
 
S

Shakey

  • Thread Starter Thread Starter
  • #8
evening chaps,
just to fill you in on a few more bits and to make things a bit clearer.

The incoming supply mis 3 phase which then uses the individual phases to feed various isolators and dis boards etc.

these are somer of the test results I did get.

ring was, L-L, 0.69 ohms. N-N was 0.66 ohms, cpc-cpc was 0.98 ohm.
L-N figure 8 1.34 ohm.
L-E fihure 8 was 1.56 ohm.
R1+R2 was 0.45ohms.
Ring Zs was 3.64ohms

Shakey, you say that the pfc is the right reading but needs to be doubled because it is 3 phase, can you tell me what difference that makes at the C/U as it is only fed with a live and neutral.

also can some of you who are more knowledgeable than me confirm I am interpreting this right,
in GN3 it states the max Zs for a type 1, 16 Amp, bs3871 CB is 3 ohms?
and for 32Amp CB it is 1.5ohms.
therefore the ZS readings I have are too high and presumably due to there being no earthing arangments???

also you say about putting a spike in for the whole system, would this then not make me "responsible" for the whole system?
my thoughts were to simply provide the earthing for the instalation I am working on and if they wish to "put right" the rest of the system then that would be a seperate issue.

I am initially working to the 16th, going on the 17th update course in couple of weeks.
could I put an rcd unit inline after the isolator to cover the whole system rather than changing both boards, advice on you ideas of instalation experience appreciated.

thanks Ross
Ross read reg 130-07-01 in the 16th

aint got in front of me but basically

"no alteration or addtion shall be carried out etc etc...... unless the earthing and bonding arrangements are also adequate"

this is no different to installing a spur in the kitchen, and having to install missing main bonds first......its just the regs

you cant just earth 'your bit', you have to earth at the origin, and yes you then take responsibility for the whole installation, because you have 'altered' the supply earthing arrangements

Imagine (for some reason) you changed a system from TNCS to TNS, then circuits which may have complied with disconnection times before may not now, and because you are the person doing the work, that becomes your responsibility

Putting an RCD covering large parts of the installation wont work either, for the same reasons as you cant do it under the 17th
 
R

Ross Trician

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  • #9
Shakey would that mean then that if I installed a suitable spike etc at the incoming supply then I would have to ensure that all the instalations on the system (not just the 2 areas I have worked on) are correctly earthed and terminated etc throughout? and that they have adequate Ze, Zs, pfc tests etc?

What do you mean about the rcd wont work for the instalation?

( apologies for the many questions but it is helpfull when you get answers from someone who knows them)
 
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SteveH37

  • Thread Starter Thread Starter
  • #10
I would suggest having a word with the farm owner and highlight the fact that the installation has a large number of defects which require attention before any additional work can be carried out. If you wish to do the work i would also have to hand a guide price for the rectification works and leave the ball firmly in his court. Sounds like the tip of an iceberg to me.
 
S

Shakey

  • Thread Starter Thread Starter
  • #11
Shakey would that mean then that if I installed a suitable spike etc at the incoming supply then I would have to ensure that all the instalations on the system (not just the 2 areas I have worked on) are correctly earthed and terminated etc throughout? and that they have adequate Ze, Zs, pfc tests etc?

What do you mean about the rcd wont work for the instalation?

( apologies for the many questions but it is helpfull when you get answers from someone who knows them)
Ross, sorry, the RCD will work, but you should not use it

Look at 314.1 (got the 17th in front of me now, but its the same as in the 16th)

"every installation shall be diveided into circuits as necessary, to:

(i) avoid hazards and minimize inconvenience in the event of a fault

(iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit

(iv) reduce the possibility of unwanted tripping of RCD's due to excessive protective conductor currentd prodeuced by equipment in normal operation)

so for these three reasons, you cant use front end RCD's, unless they have lower value RCD's downstream, (like in a TT), but under the 17th, that will mostly go anyway

in answer to you first question, the Ze is ONLY at the origin, everythin else will be Zs - and yes, you take responsibility for the lot. The pint is, to certificate your work, as you know, you HAVE to include the supply characterisitics, Ze, PFC etc, which, unless you spike at the origin (or get PME installed etc) you wont have

sorry fella!
 
R

Ross Trician

  • Thread Starter Thread Starter
  • #12
cheers Shakey and all of you for the coments and help, yes I think it is the tip of the ice burge too,
I will see if a pme is possible and if not consider options from there.

It also depends I supose if they want to do anything about it, you know the old "it's always been like it and has always worked fine" scenario.

to be honest it is quite possibly one of those jobs that you perhaps want to avoid, more trouble than its worth etc...lol.
 
S

Shakey

  • Thread Starter Thread Starter
  • #13
cheers Shakey and all of you for the coments and help, yes I think it is the tip of the ice burge too,
I will see if a pme is possible and if not consider options from there.

It also depends I supose if they want to do anything about it, you know the old "it's always been like it and has always worked fine" scenario.

to be honest it is quite possibly one of those jobs that you perhaps want to avoid, more trouble than its worth etc...lol.
yep I think its one of those as well Ross....

I have no doubt they will not want to do anything about it - but that also means that you should follow their lead!!!!:p
 
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BillR3

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  • #15
Just a word of warning if you are going to fit earth rod(s)

As you know our 4 legged friends are susceptible to electric shock more than humans due to their contact with the ground so you will have to barrier off the earth rod(s) location(s) if you end up using your TT system to keep them from danger.

BillR3
 
J

jibtech

  • Thread Starter Thread Starter
  • #16
yes possibly the funniest part of college when the lecturer explains the ground fault voltages can easily floor a heffer:eek::D

and then he explains that cows are curious beasts and the whole herd will more than likely wander over to take a look .......thud....thud...thud....thud

hahaha at 3000 quid a cow maybe more?? you better hope you can run faster than the farmer and his shotty

ps i think i would avoid this job unless the clients reaction was such that i was convinced he wants to put it totally right and proper and pay for it.cant see that happening though
 
L

lennyallover

  • Thread Starter Thread Starter
  • #17
i would suggest changing from EEBAD to ADS, stick everything on a rcbo/rcd. though some of the farm machinary will need time delays on their rcd;s.
 

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