Ross Trician

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

MacSparky

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

Get off my planet...
http://www.mysportydate.com/

Reg Man

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.

Shakey

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

Ross Trician

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

wayne

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

Ross Trician

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.

Shakey

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

Ross Trician

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)

SteveH37

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.