Design Stage

[bright_spark101]

hi all

could someone please explain to me why i need the follwing information at the design stage of an istallation -

- Prospective earth fault current (I)
- The value of external impedance (Ze)
- Assumed current demand of the installation
- Disconnection times of certain light fittings

Many thanks

 

[rebuke]

I don't have time to go into lots of detail, but essentially since they all affect the design of the system - e.g. the Ze value will dictate what type of protective device is needed/possible (for example you might not be able to use a C type, or might have to use a larger cable to get a lower resistance etc)...

 

[Taylortwocities]

Quite a number of issues to think about. these might include:

- Prospective earth fault current (I)
If this is high then you may need to spec protective devices that will withstand the PFC value. The installation may be next to a sub station eg so standard M6 breakers will die!

- The value of external impedance (Ze)
per bright spark's reply above

- Assumed current demand of the installation
You'll need to ask the DNO for a supply. Will 60A be enough. 100A? Mybe you'll need three phases to cope with the big oven, induction hob, the 3 power showers and the heated swimming pool!

- Disconnection times of certain light fittings
Your design my need some larger than usual cable csa for this...

 

[Spark123]

Homework?

Prospective earth fault current PEFC - I suppose if you were using the adiabatic for selection of protective conductors instead of selection from a chart you'd need to know this, however it is important to know the maximum fault current (Prospective Fault Current, PFC) which will flow in an installation to ensure that the correct protective device can be selected so it disconnects safely.
The PFC is the greater of the PEFC and PSCC (prospective short circuit current) - phase to neutral or phase to phase.
For example if an M3 (3000A) MCB is installed in a panel next to a substation and the PFC is 40kA then under a fault condition the MCB could explode. A correctly selected BS88 fuselink would be more suitable in this type of situation.
Under certain circumstances an MCB can be installed where the PFC is greater than that of its rating where backup protection is provided by an upstream device, you'd have to check the specifications of the two devices.

As circuits are required to disconnect within a certain time for safety the Ze is important as this added to R1+R2 for a circuit will give an indication of the Zs for the circuit. This can be compared to the max Zs for the circuit to ensure you are not going to buy a cable that is too small and won't disconnect (tho remember to compensate for temp change.)
The DNOs often cop out and just tell you the max Ze for the given supply type!

The assumed max demand is also important as you need to know how big a supply you need!

Disconnection times goes back to the selection of your protective devices again and the Zs of your circuit - if you have a 0.4s disconnection time then you need to design to accomodate this.
It isn't so much a problem now as all smallish circuits have similar disconnection times under the 17th edn regs, if you are using MCBs then providing the max Zs is not exceeded then they can be deemed to trip in 0.1s. TT supplies are slightly different as they rely on RCDs to disconnect.

edit - PSCC = prospective short circuit current

 

[plugwash]

Homework?

Prospective earth fault current PEFC - I suppose if you were using the adiabatic for selection of protective conductors instead of selection from a chart you'd need to know this, however it is important to know the maximum fault current (Prospective Fault Current, PFC) which will flow in an installation to ensure that the correct protective device can be selected so it disconnects safely.
The PFC is the greater of the PEFC and PSCC (prospective short circuit capacity) - phase to neutral or phase to phase.
For example if an M3 (3000A) MCB is installed in a panel next to a substation and the PFC is 40kA then under a fault condition the MCB could explode. A correctly selected BS88 fuselink would be more suitable in this type of situation.
Under certain circumstances an MCB can be installed where the PFC is greater than that of its rating where backup protection is provided by an upstream device, you'd have to check the specifications of the two devices.

Isn't there a rule that says provided the PFC isn't over some insane value and the service fuse is 100A or less you can assume that the service fues will provide backup protection to any MCB.

 

[GaryMo]

Yes, a BS1361 type IIb and 16kA.
You can find it in GN3.

 

[bright_spark101]

thanks for all your help guys, appreciate it

 

[ban-all-sheds]

You aren't going to be able to get through the rest of your life by asking other people the answers to all the questions you encounter.

If you cannot pass your course on the basis of your own knowledge and abilities then there is a good reason why you deserve to fail it.

 

[ColJack]

that's rather harsh BAS, we all had to learn sometimes, and having a load of facts and figures thrown at you by someone who's just trying to teach you how to pass the exam is not the best way to learn..
you look in books, search the internet and if you're still uncertain you ask guys who've done it how they do it, often they will explain in easier to understand terms.

 

[ban-all-sheds]

Or you sit there unwilling or unable to answer the question yourself, and just paste in answers that other people give you, so that it appears to the lecturer as if you are keeping up, and you are doing the work, and you do understand it all.

 

[bernardgreen]

Never criticise a student for asking, even if you think it is the dumbest of questions.

Never give the direct answer unless it is impossble for the student to find the answer from any other source.

The most effective way is to answer a student's question with another question that the student can answer. That answer being something that guides the student onto the route to find the answer to the question he asked in the first place.