Is there any way of getting the PEFC if you only have the Ze?

[Ashley2]

Is there any way of getting the PEFC if you only have the Ze?

 

[D Skelton]

(Post edited after reading OP's past posts)

Ohms law.

The Ze is the external earth fault loop impedance (resistance for simplicities sake)

Divide the voltage (230V) by the Ze and you will get the potential current flow (prospective earth fault current)

I=V/R

Someone may correct me here but I believe that the PFC should always be measured, not calculated.

 

[Ashley2]

I would have thought that the PFC is always measured (Taking the higher of the PEFC/PSCC), & then a calculation to ensure that the protective device is able to handle the PFC.

Some of the information given was as follows:
The Ze given was 0.8Ω
The voltage given was 230 V a.c.
The protective device was 100A.

I am trying to remember the above. I am positive about the voltage & protective device, & about 90% positive about the Ze figure.

I initially thought it was Ohms law, but the figures did not tally with that, i.e. PEFC should be less than the 100A. This is why I asked whether there was a way of calculating the PEFC with the Ze given.

 

[telectrix]

it is ohm's law I = V/R, so in the case you mention. I = 230/0.8 = 287.5A. what makes you think it should be <100A. the rating of the fuse has nothing to do with it.

 

[scotsparky]

It all depends on the earthing system

In a TN-C-S the PEFC and FSCC current will be the same
In a TN-S and TT system they will be different ats the Neutral and Main earth are taking different routes

Always just measure and I put down the lowest result taken as thats the worst case

 

[D Skelton]

I would have thought that the PFC is always measured (Taking the higher of the PEFC/PSCC), & then a calculation to ensure that the protective device is able to handle the PFC. This is correct

Some of the information given was as follows: Information to what?
The Ze given was 0.8Ω
The voltage given was 230 V a.c.
The protective device was 100A.

PEFC should be less than the 100A. This is why I asked whether there was a way of calculating the PEFC with the Ze given.

On a TN system, PEFC will never be lower than 100A. The lowest possible PEFC on a TN system is on a TN-S with a value of 287.5A

On a TT system PEFC will more than likely be lower than about 2A and between 2 and 10A if it's a really good TT system (domestic)

I must admit, I am slightly confused. Are we on the same wavelength?




Edit:

it is ohm's law I = V/R, so in the case you mention. I = 230/0.8 = 287.5A. what makes you think it should be <100A. the rating of the fuse has nothing to do with it.

Beat me to it

 

[Ashley2]

The 100A information was already given, for the protective device, as was the Ze for the supply. And the question asked what the PEFC was. Bear in mind that this was a part B scenario for 2391, & the 100A for the protective device was already given, therefore, in the first instance, the PEFC must be less than the protective device. This is why it became questionable, in relation to calculating, using ohms law.

 

[D Skelton]

The 100A information was already given, for the protective device, as was the Ze for the supply. And the question asked what the PEFC was. Bear in mind that this was a part B scenario for 2391, & the 100A for the protective device was already given, therefore, in the first instance, the PEFC must be less than the protective device.

If the PEFC was lower than a protective device, it would never operate during an earth fault. Hence the reason why (rightly or wrongly so) RCD's are relied upon to provide earth fault protection in TT systems.

PEFC is calculated using voltage and Ze only, forget the info about the 100A protective device because it seems to be confusing you and throwing you off.



If what you're worried about is thousands of amps flowing through a OPD rated at only 100A, you're concerned about the wrong rating. Assuming the protective device is a 60898, it will be rated at 6kA or above for fault conditions.

 

[Ashley2]

I am looking at 435.5.1: which states that the breaking capacity rating of each protective device shall be not less than the prospective fault current at its point of installation". I strongly suspect that it is ohms law, but the 100A protective device was the mind boggler, taking into account 435.5.1.

 

[D Skelton]

I am looking at 435.5.1: which states that the breaking capacity rating of each protective device shall be not less than the prospective fault current at its point of installation". I strongly suspect that it is ohms law, but the 100A protective device was the mind boggler, taking into account 435.5.1.

The breaking capacity of a breaker is not the same as its rating. It is rated at 100A but its breaking capacity will more than likely be 6000A or higher if it's a BS60898 or around 2000A if it's a BS3036

 

[Ashley2]

Yes, its not the 100A protective device, its the Icn value, "& for the majority of applications the prospective fault current at the terminals should not exceed this value". It is ohms law, KISS should be the motto in 2391.

 

[IQ Electrical]

Yes, as D Skelton points out, you're confusing the overcurrent rating with the short circuit capacity of the device.

 

[1shortcircuit]

the PEFC must be less than the protective device

Correct, look at the side of a 60898 and on the side it will say 6000. Meaning that device will take UP TO 6000amps of fault current before being blown into pieces which would obviously create a lethally dangerous situation.

 

[Farmelectrics]

measurement is better than calculation lol

 

[telectrix]

if the OCPD is a fuse to BS1361 or BS88, or whatever they change the BS number to tomorrow lunch time, the breaking capacity is in the region of 16KA or 33KA.