generators

[richy3333]

Can he not use the anchor as a means of earthing?

 

[fadox21]

Enginner54 I did not say that a earth was not required at the neutral star point of the genny. I know it is earthed to the can of the genny and then on to a ground rod. As my statment read i have just started working with generators and have been informed that on a tncs system a ground rod was not always required but i know the neuteral always is earthed to the can. Also if you look in the regs at the generator system diagram the earth line to the ground is shown as opptional per system. I do not proclam to be an expert with generators and am glad you have pulled me on these comments I was just giving my view from what i have been shown. Are you saying all normal site tncs generators need a ground rod? also if you know any reg numbers or other type of legistation which I could look at to educate myself more on these systems would be helpful.

 

[Guest123]

also if you know any reg numbers or other type of legistation which I could look at to educate myself more on these systems would be helpful.

Have a look here, issue 39, summer 2011, portable generating sets.

Wiring Matters - magazine for electrical installation contractors - IET Electrical

 

[Engineer54]

Enginner54 I did not say that a earth was not required at the neutral star point of the genny. I know it is earthed to the can of the genny and then on to a ground rod. As my statment read i have just started working with generators and have been informed that on a tncs system a ground rod was not always required but i know the neuteral always is earthed to the can. Also if you look in the regs at the generator system diagram the earth line to the ground is shown as opptional per system. I do not proclam to be an expert with generators and am glad you have pulled me on these comments I was just giving my view from what i have been shown. Are you saying all normal site tncs generators need a ground rod? also if you know any reg numbers or other type of legistation which I could look at to educate myself more on these systems would be helpful.

Talking generally here now, not about Marine systems, that i've since found out, use an insulated neutral arrangement, similar to a double fault IT supply source.

To be absolutly honest, i've yet to see a TNC-S system derived from a site generator, probably because it doesn't make sense to do so!! What i will ask you, is How do you intend to earth the Genny neutral, without a ground electrode then?? Connecting the neutral to the can/frame, isn't earthing the neutral, unless the can/frame is earthed itself!!

Do you think that the DNO's local distribution TX's are not solidly grounded at the neutral, whether they be supplying TN-S or TNC-S installations...

If your talking about tiny sub 5KW portable Genny's, then they just rely on a floating neutral, that's fine for running site tools and a portable lights around a building site etc, but not much else....

 

[robbie18]

the lads are coming back with Zs readings around about the 3 ohm mark and the 25amp RCBO max Zs is around 0.72ohm
If i am reading the regs right the 1667ohm would be for the rcd part to operate correctly but anything over the 0.72 would fail for over current protection? possibly i am reading and understanding this part of the regs wrong?

 

[spark 68]

Hi Robbie,

If i am reading the regs right the 1667ohm would be for the rcd part to operate correctly but anything over the 0.72 would fail for over current protection?

You are mixing two different fault current paths up here, the 1667 ohms is max Zs , which is for the RCD part (if it is a 30mA device), which is an Earth fault path, which the RCD part takes care of for you.

The overcurrent/Short circuit fault path will be Phase to Phase, or Phase to Neutral, which as long as the OCPD rating (the 25A part) does not exceed the PSCC output capability of the generator, or at the circuit itself , will be fine.

 

[spark 68]

Just and addition to above,^^^
You will need to measure your R1 + Rn, to see if this is less than your stated 0.72 ohms, also you may find that you only need a 1 or 5 second disconnection time as opposed to a 0.2 or 0.4, depending on the circuit arrangement and type of earthing system, this may then allow a higher permissable max reading with regard to overcurrent/Short circuit protection.

 

[robbie18]

So could someone tell me if max zs for 25amp RCBO is 0.72ohms and test results are 3.4 ohms is this a pass or fail? I would take this as a fail all day long!

 

[spark 68]

So could someone tell me if max zs for 25amp RCBO is 0.72ohms and test results are 3.4 ohms

As already said, anything below 1667 ohms for a 30mA RCD/RCBO is a pass for Zs (Earth fault Impedance)

 

[robbie18]

So what ate the max zs ratings for RCBO's there for then? Surely the 0.72ohms must mean something?

 

[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.