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.

 

[robbie18]

So I am testing ZE at 0.10 and then ZS at end of circuits at 3.5 protected by 25amp type c RCBO this is a pass??

 

[fadox21]

engineer54 I have always thought a tncs system worked your way but as i have never worked beyond an installation ie in the network so have not had any experience of working with to many differnt systems. When I started my new job working along side generators two sparks there explained to me that an earth rod was not always needed on a tncs system. I did question this but eager not to look unexperienced and as they both had experience in this field and also ship wiring which uses no earth rod, I assumed I had miss understood the system. Since you pulled my comment i have found as many sources of information as I can and educated myself reguarding tncs sytems. I am happy to have been wrong as otherwise I would still be non the wiser. I have explained to these sparks and concluded that they some how had the IT system for ships and a TNCS system mixed up and that a source of earth was always required for a TNCS. Reguarding the generators most already come with a neutal earth link inside with a label on the front stating this. All the generators have rods installed, it was just the fact I was told that if removed the ocpd would still work fine, which we can safely say I know now to be wrong. Anyway thanks for pulling my comment and I think ill keep my advice to myself unless 100% sure next time, oh well

 

[Engineer54]

Well we have both learnt something from this thread then...lol I now know that marine electrical systems, use an insulated neutral arrangement...lol!!

Always better to ask questions, or ask to confirm your understanding of things, than keeping quite. Nothing stopping you giving advise on these forums, if your wrong you'll get someone here that can correct you, ...just another way of learning and picking up things!!

 

[spark 68]

Robbie what is the trip rating in mA of the RCD part of your RCBO ?

The max Zs values for RCD's RCBO's depend on the mA rating, and are as follows:

30mA = 1667 ohms
100mA = 500 ohms
300mA = 167 ohms
500mA = 100 ohms

So your Zs values will be fine


The OCP part requires more thought, as your Ze values are low, it is the (R1 +R2) values which are giving the higher part of your readings.

If it is a TN system and a distribution circuit, a 5 second disconnection time will be allowed.

As a 25 A C type OCPD will require 250 A to operate instantaneously, 10 X 25A , 230/250 = 0.92 ohms, but if you are allowed a disconnection time of 5 secs this can be higher.

Edit; I have not included any correction factors here for clarity

 

[robbie18]

It is a 30ma device we use! Thanks for all your posts so far! I am just not too familiar with these sort of systems and results!!