Quote:
Originally Posted by 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 |
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