Prospective fault current
- Thread starter Jacobssi
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If you are studying for 2391 or any of the Inspection and Testing exams then you should be aware that as well as the maths, the prospective fault current value is the greater of either the prospective short circuit current or the prospective earth fault current.
The amount of current that would flow until operation of the protective device.
This could be due to a direct short between L and N, or L and E, or phase to phase on a three phase install.
The fault current would be determined by the loop impedance of the fault and the supply voltage, for example, if your Ze at the origin is 0.15ohms, your prospective fault current would be 230 devided by 0.15 which equals 1533amps (1.53kA). Some beginners tend to assume that because the main fuse is 80 or 100amps that the current that will flow during a fault will be limited to this - it wont be. Thousands of amps will flow until the protection operates (which is typically in a fraction of second).
The PFC would decrease the further down the circuit or installation you go due to the loop impedance increasing.
This could be due to a direct short between L and N, or L and E, or phase to phase on a three phase install.
The fault current would be determined by the loop impedance of the fault and the supply voltage, for example, if your Ze at the origin is 0.15ohms, your prospective fault current would be 230 devided by 0.15 which equals 1533amps (1.53kA). Some beginners tend to assume that because the main fuse is 80 or 100amps that the current that will flow during a fault will be limited to this - it wont be. Thousands of amps will flow until the protection operates (which is typically in a fraction of second).
The PFC would decrease the further down the circuit or installation you go due to the loop impedance increasing.
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in simple terms.
it's how many amps can flow in a dead short between the Live and Neutral, or the Live and earth at the CU isolator.
whichever works out the largest is the one you write down.
it's how many amps can flow in a dead short between the Live and Neutral, or the Live and earth at the CU isolator.
whichever works out the largest is the one you write down.
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I understand that fuses have a let through value which is the amount of current that will rupture them without delay.
So even if the fault current is over 4700A if the let through value is below that figure one can still use protective devices which are marked as 4700.
The I²t value is taken from manufactures data. The time is all to do with ensuring the fuse ruptures before any circuit breaker tries to open so both fuse and circuit breaker information needs checking.
I have only once found a problem with the 4700A value on a RCBO when taking the supply from the main 400A moulded breaker feeding a crane to also feed a 13A socket to charge the crane to banksmans radio. Fitting a 16A fuse in the main distribution unit with 2.5mm² cable feeding the box used for radio by time it entered that box it was within the 4700A value for double pole RCBO and so I think in reality this is only a college exercise and in practice one does not need it.
Otherwise all answers are 100%.
I must admit it does seem strange that a RCD which is not going to trip on over current should be rated both with max fault current and max continuous current as it would be impossible to calculate the let through value. However since unlikely the max fault current would ever be reached it is academic. Larger supply RCD protection is normally combined with the moulded breaker anyway so likely it will not be a problem.
However this is a DIY forum and questions about this should really be on a professional forum like IET.
As to measuring PFC I have seen many meters which measure only the line - earth value. Only the top of range meters measure both values and display the highest readings and care is needed to know how to use the meter one has since they are not all the same.
This is only when using the three core plug and socket lead of course using standard wandering leads selection is of course up to user.
So even if the fault current is over 4700A if the let through value is below that figure one can still use protective devices which are marked as 4700.
The I²t value is taken from manufactures data. The time is all to do with ensuring the fuse ruptures before any circuit breaker tries to open so both fuse and circuit breaker information needs checking.
I have only once found a problem with the 4700A value on a RCBO when taking the supply from the main 400A moulded breaker feeding a crane to also feed a 13A socket to charge the crane to banksmans radio. Fitting a 16A fuse in the main distribution unit with 2.5mm² cable feeding the box used for radio by time it entered that box it was within the 4700A value for double pole RCBO and so I think in reality this is only a college exercise and in practice one does not need it.
Otherwise all answers are 100%.
I must admit it does seem strange that a RCD which is not going to trip on over current should be rated both with max fault current and max continuous current as it would be impossible to calculate the let through value. However since unlikely the max fault current would ever be reached it is academic. Larger supply RCD protection is normally combined with the moulded breaker anyway so likely it will not be a problem.
However this is a DIY forum and questions about this should really be on a professional forum like IET.
As to measuring PFC I have seen many meters which measure only the line - earth value. Only the top of range meters measure both values and display the highest readings and care is needed to know how to use the meter one has since they are not all the same.
This is only when using the three core plug and socket lead of course using standard wandering leads selection is of course up to user.
what the hell is a moulded breaker?
the main fuse if there to protect the cable from the main fuse to the meter then on to the main isolator and busbars in the CU all the way to the breakers / fuses.
any shorts downtream of the breakers will operate the breaker long before it operates the main service fuse.
the main fuse if there to protect the cable from the main fuse to the meter then on to the main isolator and busbars in the CU all the way to the breakers / fuses.
any shorts downtream of the breakers will operate the breaker long before it operates the main service fuse.
P
PrinceofDarkness
I suppose the O/P as a 'learner', will find the above definitions/explanations useful. But having a simplified definition is no substitute for learning (and understanding) the actual definition - as issued by the Supply Industry - and more importantly: the reason for having such information........
"PSCC: The current that would flow in acircuit, in the event of a short circuit of negligble impedance, if the overcurrrent protective device were replaced by a conductor of negligble impedance; it is measured as the RMS value of the ac component. The actual fault current will therefore be less if the protective device has a current limiting feature or an appreciable impedance".
Lucia.
"PSCC: The current that would flow in acircuit, in the event of a short circuit of negligble impedance, if the overcurrrent protective device were replaced by a conductor of negligble impedance; it is measured as the RMS value of the ac component. The actual fault current will therefore be less if the protective device has a current limiting feature or an appreciable impedance".
Lucia.
Site slang for a MCCB I guess!
Edit:
Though those who work on installs from the 60s might be familar with items of switchgear branded as ECC moulded breakers which appears to have some relation to the Federal brand (certainly the ECC DBS were full of FPE CNA type 4 stab-lok breakers)
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That is in effect PEFC (prospective earth fault current), however it should be noted that PEFC should be measured with the protective equipotential bonding in place whereas Ze should be measured in isolation.
Prospective fault current PFC is the greater of the PEFC or PSCC (prospective short circuit current), the PSCC in a single phase install being between L&N.
In a 3 phase installation you would need to measure each phase to N value then if your tester can do it measure phase to phase values. PSCC is the maximum value. If your meter wont do phase to phase then you just double up the maximum phase to neutral value.
Patronising Git. So the only people registered on here are DIYers?
Maybe this is because when the fault current flows, lets say 5KA, it's going to be flowing through the RCD, regardless of whether the RCD can sense overcurrent or not, and it will probably blow the s**t out of it's internal workings.
Or is that too simple a suggestion for you?
And as you seem to be saying a similar thing to PrinceofDarkness with regards restricting fault current, we'll pick on him:
How is an MCB not 'of negligible impedance'??
How is it limiting the current or having an 'appreciable impedance'
Would this not show up on your Zs tests?
You know - Ze = 0.35.........R1 + R2 = 1.6.........Measured Zs = 1.85
Oh Oh, where's our MCBs 'appreciable impedance' or current limting feature??
P
PrinceofDarkness
I don't understand your problem, Electifrying. The definition I provided was in quotation marks and was quoted verbatim from The Electricity Supply Industry's official definition.
There's only one way of defining the PSCC, and that's the 'notional' figure of the maximum value of fault current that would flow if if were not for the limiting devices (such as MCBs) in the circuit.
Given that, in actuality, there are MCBs/fuses etc in the circuit, then the PSCC is somewhat attenuated. But the fact remains that the Supply Industry calculates that notional figure as though there are no limiting devices (negligble impedance) which is the only logical way of doing it.
So, don't split hairs with me until you have a better understanding of the supply industry, dear. Otherwise, take it up with the ESI.
Lucia.
There's only one way of defining the PSCC, and that's the 'notional' figure of the maximum value of fault current that would flow if if were not for the limiting devices (such as MCBs) in the circuit.
Given that, in actuality, there are MCBs/fuses etc in the circuit, then the PSCC is somewhat attenuated. But the fact remains that the Supply Industry calculates that notional figure as though there are no limiting devices (negligble impedance) which is the only logical way of doing it.
So, don't split hairs with me until you have a better understanding of the supply industry, dear. Otherwise, take it up with the ESI.
Lucia.
If you had a Ze of 0.03 (say inner city, close to large TX), would you be concerned that your domestic CU was only rated to 6kA?
How about a private sub on an industrial site with a Ze of 0.01 or less - would you be concerned that your final MCB's in a board close to the origin where only 10kA?
This is where backup protection and let through energy comes into play.
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