generators

[spark 68]

I looked through the previous posts and cannot see any mention of 0.72, where did you get this figure from ?

 

[robbie18]

This is just a rough guess at max zs for a 25amp RCBO from the regs tables! Type C

 

[spark 68]

Again you are mixing max Zs up with Over current protection.

Your Earth fault current is taken care of by the RCD part, which allows up to 1667 ohms for a 30mA device

Zs = Ze + (R1 + R2)

 

[robbie18]

So what is the point on having that table then?? I am not used to testing where zs is higher than this table without it being a fail?

 

[spark 68]

The OCP part follows a different path, being either R1 + Rn or R1 + R1, either phase to neutral, or phase to Phase, Ze does not play a part in this. You will probably find the fault path impedance is a lot lower (with no Ze to add).

Your EFLI path is taken care of by the RCD part.

The tables also relate to where an RCD is not used, as is the case in many applications.

 

[robbie18]

So are you saying that if you use an RCBO and RCA test pass fine and zs readings are below 1667 then it's a pass??

 

[spark 68]

Think for a moment why we fit RCD's to a TT system which may have a Ze of up to 200 ohms

 

[robbie18]

As I said and the start of this posts,I am unfamiliar with these type of readings so wanted some experienced people to inform me,so these readings are acceptable that I am getting??

 

[spark 68]

I will try to explain a bit.

Zs = Ze + (R1 +R2)

For instance a lighting circuit protected by a B type 6A MCB

If we had a Ze of say a 100 ohms, and an (R1 + R2) reading of 1 ohm.

A type B6 amp MCB requires 5 X 6A to break instantanously (5 being the multiplier for a B type device).

230/(5X6) = 230/30 = 7.6 ohms (max Zs)

Zs = Ze + (R1 + R2) = 100 + 1 = 101 ohms for our Zs value, clearly this value is far too high to trip the B6 MCB, so by adding a 30mA RCD, this will increase the max Zs allowed up to 1667 ohms, our circuit above will then trip to give the required disconnection time.

 

[spark 68]

Using the above values for Short circuit/over current protection, assuming the same size CPC as our line conductor.

Our fault path this time being R1 + Rn, (R1 + Rn) = 1 ohm (which is a far lower reading without the much higher Ze), which is lower than the 7.6 ohms allowed, and so the MCB would trip in the required time.

Using an RCD does not replace the need for low (R1 + R2) values, but does mitigate the higher Ze readings.