generators

[fadox21]

In regards to an earth rod, ive just started working with generators especially tncs systems. I have found that an earth rod is used mainly as a lighting protector only and that any faults with the systems go to the star point on the transformer via the neutral. Meaning a rod is not always nessersary just like a ship as you said. Myself I would check all the earth connections and also the pen link at the genny. What is your ze at the genny? is that a normal reading

 

[robbie18]

Yeah Ze is coming back fine, Like i said before i am new to the company and just wanted to try and get some info on these systems as i am new to them, it does not say much for the electricians before me or below me though as they dont seem to mind high Zs readings. All problems seem to be is length of run and size of cable but just wanted to see if anyone could give hints or tips about working/testing when generators are involved.

 

[Deleted member 26818]

You keep refering to high Zs, but you don't actually state what the Zs readings are.
Are they greater than 1667Ω?

 

[Engineer54]

In regards to an earth rod, ive just started working with generators especially tncs systems. I have found that an earth rod is used mainly as a lighting protector only and that any faults with the systems go to the star point on the transformer via the neutral. Meaning a rod is not always nessersary just like a ship as you said. Myself I would check all the earth connections and also the pen link at the genny. What is your ze at the genny? is that a normal reading

Are you sure you know what your talking about??
The earth rod is mainly there as a lightning protector!!! No need to earth the neutral star point of the generator??

Who the hell's been feeding you this Crap??

 

[general error]

Hi Robbie
The big difference between Mains & Generator is maintenance of the fault current.
Generators will not maintain fault current the way mains does.
This is probably the limiting factor for effective operation of any protective devices
The design information may be available from the manufacture, if not, you need to decide whether you want to establish it by experiment.

I probabley would not touch this job but when I was younger I would be minded to consider the barges as portable appliances, use an earthless system and have multiple redundancy on the RCCDs for protection against shock.

If the load is aeration pumps that draw a significant part of the generators output You may not be able to protect against short circuit by any method of automatic disconnection available.

 

[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?