Help to understand pfc pscc values

[Farmelectrics]

Can some one explain if your pscc value is say 4.6ka well in gn3 it says that your protective device must not be less than that value so say the protective device was 5ka how would the protective device trip if it's fault current was only 4.6ka please explain cheers lads

 

[telectrix]

the 5KA is the breaking capacity of the PD. i.e. the max. fault current it will withstand without breaking down. look at it similar to a 100A main switch. you don't have to run 100A through it.

 

[Farmelectrics]

Thanks telectrix was struggling to get me head around it I understand

 

[Farmelectrics]

Telectrix may be I don't fully understand surely if there was a fault current you would want your pd to break

 

[TaffyDuck]

Telectrix may be I don't fully understand surely if there was a fault current you would want your pd to break

If you look at the side or the front of most MCBs you will see a rectangle with a figure in it. Usually 6000 on modern MCBs. This is the max fault or short circuit current the MCB can handle before it kills the MCB or it fails to work.

The confusion is the word breaking. Tel meant busting the pd.

 

[Farmelectrics]

Taffy and telectrix thanks for your advice but still strugling so if I have a fault current of 4.2ka and the device breaking value is 5ka how would the protective device trip

 

[telectrix]

device will trip at way under the KA rating. look at the time/current graphs in app.3 of BS76571. for arguments sake, a 32A type B MCB with a fault current of 100A will trip in 90secs. the breaking capacity is 6kA, that is the max. fault current it will withstand without either arcing over or self destructing. in other words, MCBs and fuses will trip according to the level of overload. the larger the fault current , the quicker they will trip. you seem to be getting confused between breaking and tripping. the PD will break a fault current of up to 6KA ( or whatever it's KA rating is)

 

[ringer]

farmelectrics, look at the front of a MCB. It should have two sets of specification information (beyond BS numbers and manufacturer's ID info). A typical example might be B6A (indicating a 6A breaker as typically used for overcurrent protection on a domestic lighting circuit) and 6000 (or 6kA) (indicating the maximum withstand current under fault conditions). The breaking capacity of 6kA may be written on the side of the MCB, or it may, for example, be presented as 'M6' (on MEM devices)
So we are looking at two different things here - overcurrent protection for the circuit and the fault current capacity of the device that provides the overcurrent protection.

Typical examples:

A lighting circuit is overloaded with light fittings. All lights are switched on. They draw 8A of current (yes, a lot of lights, but not impossible to achieve!). The 6A MCB protecting the circuit will (eventually) trip.

Alternatively, a lamp blows. There is a momentary short circuit of the lamp element, and a large current is drawn. Perhaps in the order of 100A. This will trip the MCB.

In both of the above cases, the MCB, which is rated to be able to withstand a fault current of 6kA, will operate and can be safely reset and used again.

It also helps if you understand that fault current rises, it is not instantaneously at its maximum. The longer a fault is allowed to exist, the higher the current rises. In the second example above, the fault existed for perhaps a few milliseconds before the element disintegrated.

Another scenario - a line conductor comes loose and touches an exposed conductive part of the installation. This fault continues for seconds, minutes even. Potentially huge earth fault current ensues (4.3kA in the example you quoted). If this fault current is less than 6kA then the MCB will safely trip and can be reset and reused.

If the fault current was 8kA then it is possible that either the MCB will just melt and the contacts will be welded together so that the MCB is unable to trip, or the MCB may actually explode and could harm persons nearby or could start a fire.

What limits the fault current is the Earth Fault Loop Impedance. If the impedance is too low (yes, low impedance can be a bad thing!) then the fault current would be too high. We need to design our circuits and select our equipment to maintain an acceptable balance. On the one hand, we want to encourage fast-rising fault currents (by keeping Ze and Zs low enough) but we do not want Ze and Zs so low that the fault current rises above the withstand capability of the protective devices used.

I hope that this, along with Tel's description above, helps you now understand the difference between the two specifications.

 

[Farmelectrics]

thanks lads the penny has dropped what a great site this is

 

[Engineer54]

What limits the fault current is the Earth Fault Loop Impedance. If the impedance is too low (yes, low impedance can be a bad thing!) then the fault current would be too high. We need to design our circuits and select our equipment to maintain an acceptable balance. On the one hand, we want to encourage fast-rising fault currents (by keeping Ze and Zs low enough) but we do not want Ze and Zs so low that the fault current rises above the withstand capability of the protective devices used.

This is one of the most difficult but very important aspects of power distribution design, be it by DNO or on private estates/sites. Getting the ratio/balance right during the design stages of a project, is a lot more difficult, than first meets the eye!! And needs updating and implementing for any site/project changes that are made, be they additions or reductions. Literary every main switchboard, sub-switchboard and final DB's and there contents will be purchased rated and or based on these design calculations... Not to mention cable sizing!! lol!!