Breaking capacity

[Richard3009]

With respect to PSCC what is meant by the breaking capacity of the protective device? what figure are they talking about

 

[ryanryan]

in the event of a short circuit a massive fault current will flow.
the breaking capacity of a protetive device must be capable of interupting the fault current.
Most domestic mcbs are 6000 amp, some 10000.
indicated in a box on the mcb.

 

[Richard3009]

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

 

[zupos40]

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

Read osg page 57 or GN3 pages 52 & 53 for examples

 

[Richard3009]

Protective device at origin of installation, is what? what is classed as origin

 

[zupos40]

Protective device at origin of installation, is what? what is classed as origin

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

 

[scotsparky]

my understanding is the breaking capacity id the maximum fault current the mcb will take and still be able to be reset.

 

[Richard3009]

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.

 

[zupos40]

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.

No rcd are not installed to provide protection against PFC, the electricity suppliers fuse and the MCB will provide protection against PFC (Fault L-N)

The RCD are their to detect earth faults (L-E or N-E) all RCD have a rated short circuit capacity of about 1kA only.

So when you are performing PFC testing the rated short circuit capacity of both the supplier fuse and your fuse/ circuit breakers installed in your CCU should be capable of disconnecting this current under L-N fault condition, not the RCD.

 

[electricAl]

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

Good generic description for you by Zupos, but bear in mind not all installations will be upto recent specs included here - tails may be smaller, may feed more than one CU or you may find a 60A cut-out (common on bungalows etc)

The origin would be considered as at the DNO`s main (cut-out) fuse. All before that is regarded external to the installation (Suppliers equip / Ze) whilst all downstream is internal part (Consumers / R1+R2)

my understanding is the breaking capacity id the maximum fault current the mcb will take and still be able to be reset.

Icn = SC rating of protective device (marked in amps, in the little box) Fault current it can safely withstand without exploding/welding contacts etc - would not be expected to be usable afterward

Ics = service rating (the other single digit representing KA) same as above but the fault current it can withstand but still be serviceable after being reset

Cheers

 

[Richard3009]

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).

 

[electricAl]

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).

You`ve got it. Max fault current is the inverse of the loop. So it reads resistance of the loop, then divides into the measured voltage to derive PFC.

Basic Ohms... PEFC = Uo/Ze & PSCC = Uo/Zln

therefore Ze = Uo/PEFC & Zln = Uo/PSCC

The readings should be ideally taken as close to the meter as possible - for PEFC take phase from incomer (isolator open) & Earth from MET (or earthing bar/ link at supply cable)
Obviously, the PSCC measure would be taken between L & N (for single phase) at supply side of main switch.

Hope that helps

 

[Richard3009]

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? 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,

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,

 

[electricAl]

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...

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

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 )
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

 

[Richard3009]

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,