R
Richard3009
With respect to PSCC what is meant by the breaking capacity of the protective device? what figure are they talking about
Discuss Breaking capacity in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net
Thanks thats exactly what i needed, so it's marked on the mcb sorry to be a pain but im not one for doing test and putting numbers in boxes without knowing exactly what to check the result against.
Just getting as much knowledge as I can before doing my 17th
Protective device at origin of installation, is what? what is classed as origin
belonging to you the householder as installed in the CCU, but it could be the REC cut-out
Table 2.4 in the Guidance notes doesnt mention an RCD ?????? what is the first protective device on a 17th edition compliance CU it cant be the main switch.
Outside in the street would be a local transformer on the output of that transformer would be connected a 3 phase supply which singlephase would be taken off to supply each house as the cable enter the house it terminated in to a REC service head which will have either an 80A or 100A BS1361 or BS88 fuse, from this fuse will doulble insulated 16mm or 25mm tails to the electricity supplier metering from the metering double insulated single core cable feed a CCU. I believe The origin is the first protective device belonging to you the householder as installed in the CCU, but it could be the REC cut-out
my understanding is the breaking capacity id the maximum fault current the mcb will take and still be able to be reset.
When using an instrument, what exactly is it measuring? I assume the resistance of the fault path, is this measurement taken at the incoming supply terminals with the CU main isolator closed (live).
So using a 3 wire test, two of the wires would be to measure the voltage and the third to measure the resistance of the earth loop? [/qoute]
No, Richard. If using a 3 wire test meter then the usual procedure would be to connect the green & black crocs together, effectively making it a 2 wire tester for PFC (but check out user instructions for exact meter) Then, it`s as described above for 2 wires...
Again, no my friend, for PFC readings the earthing/bonding should all be in place. We`re trying to establish the maximum fault currents that could effect operation of protective devices & ensure that they`re upto the job (despite the content of the `other` post on this subject )is the main incoming earth removed from the earth bar for this test; and croc clipped to measure impedance of the earth path back to the supply transformer
Be clear - we need the install to be as it would be under fault conditions - so as it would be in normal operation before the fault occurs. This would necessitate leaving the earthinng/ bonding in place when measuring, as the `normally` available parallel paths would induce a greater Earth Fault current, & it`s this `maximum` current we need to measure.
It`s perfectly fine (often more convenient) to croc clip to earthing bar to take PEFC - sorry if my previous post misled you on that - i`ve amended it
Surely the possible size of current that can be drawn prior to disconnection is determined mainly by time, and this will be determined by the protection device installed by the supply company, anything from a 60 to100A cartridge fuse
how can a test instrument get a measure of this,
You`re half right on that one. The cut-out fuse is there to do as you say. Trouble is, that fuse has it`s limitations - too much current drawn & it could disintegrate, explode or otherwise fail to interrupt the fault current - exactly what we don`t want after dutifully doing all we can to encourage the fault current to flow thru the install
So it`s critical we establish that the largest possible fault current that could be supplied is not greater than the protective device(s) can safely handle. Hence our measurement of the impedance of the fault path & the following calculation to estimate PFC
ok so the installation is left in its normal state energised, the bit that confuses me is 2 readings are taken and the highest recorded, what terminals are the two readings taken from.
Phase to Neutral followed by phase to earth and the larger of the two readings recorded,
Yep got that the test is clear now, what is the state of the main CU isolator during the test
I switch off all cbs or remove fuses then switch off the double pole main switch and lock it off, before I remove the main earth to measure Ze, then reconnect earth measure Pefc (L-E) and then finally PSCC (L-N) only then do I remove lock switch on main switch, then each cb/fuse.
hang on you are both saying different things one has an open DP switch the other has it closed
Its nothing to do with risk Im wondering about the low readings through lamps
Then just how would you be able to take a PSCC with the install isolated? That is the approved procedure.
surely the impedance between L and N is effected by a load like a lamp in the circuit when taking a measurement.
Yes I realise that, my question is if the switch was closed would it impact the reading, sorry if Im being padantic but I like to FULLY understand things
next continuity
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