generators

[robbie18]

Does anyone have any experience of testing when generators are involved? All ZS readings come out too high for RCBO's that protect the circuits! any help or advice would be greatly received.

 

[Knobhead]

What are the earthing arrangements for the generator?

You need to give more information.

 

[telectrix]

not too well up on genny supplies, but assume a TT earthing system, so max. Zs would be 1667 with RCBOs.

 

[robbie18]

TN-C-S earthing arrangement, these generators are used on barges at fishfarms which might also be useful information

 

[MarkieSparkie]

Are you sure your earthing arrangement is TN-C-S, ESQCR:2002 prohibits the connection of a PME earthing facility to any metalwork on a boat.

 

[robbie18]

to be honest i am not 100% sure on this. all i find is that when i am looking through my new companies test sheets that all zs readings are too high for RCBO's that are protecting the circuits. Are generators usually TNS earthing arrangement? Cables are run under the water so operating temperatures and everything are hard to determine. Company have the thought that if circuits are protected by RCBO's then they have done enough to which i would like clarification on.

 

[Deleted member 26818]

What are the ratings of the RCBOs?
If they're 30mA then as Tel says the maximum Zs allowed is 1667Ω.

 

[Engineer54]

to be honest i am not 100% sure on this. all i find is that when i am looking through my new companies test sheets that all zs readings are too high for RCBO's that are protecting the circuits. Are generators usually TNS earthing arrangement? Cables are run under the water so operating temperatures and everything are hard to determine. Company have the thought that if circuits are protected by RCBO's then they have done enough to which i would like clarification on.

You don't seem to be that clued up on generator supplied systems. So what's the KVA size of this genny, has the neutral been solidly connected to earth, what sort of earthing arrangement does the generator have, single or 3 phase?? A poky sized single earthing rod, isn't going to cut it, on any genny install that's not of the portable variety....

 

[robbie18]

kva sizes are mixed but one we use is 150kva, neutral connected to earth at terminal box,3 phase generator.earth runs from generator on to metal structure of barge. barge sits in the middle of a loch with no earth rods or anything like that. hopefully someone can shed some light on this issue as i am new to generator installs as yous have probably guessed

 

[Knobhead]

No earth rods? This gets worse.
So the only earth connection is floating about on the water on the end of a flexible cable.
You really need to step back from this.

 

[robbie18]

obviously there are no earth rods, it is a barge in the middle of a loch. a ship can not have earth rods as it sails so i dont think this is an issue. The problem i am having must purely be down to cable lengths and cable sizes i am sure but my original post was just looking for anyone who has had experience of this environment

 

[Knobhead]

Cables are run under the water so operating temperatures and everything are hard to determine.

I was going on this.

 

[robbie18]

what we do is run power to the cages on the fishfarm so by that i mean from cage to cage the cables are under the water. the bottom line seems to be that my new company have been running to small a cable for these installations hence zs values are coming back high

 

[Knobhead]

I’m not even sure water bourn generators come under BS7671. Durham is the one to ask. From what I’ve read ships are usually earth free with monitoring systems. I’ve dealt with land based earth free systems and to be honest they were a pain it the butt.

 

[Engineer54]

Like Tony, i've no experience of shipbourne generators earthing arrangements!! I think it make some difference whether the barge is floating in sea water or fresh water though. but i wouldn't want to swear on it ..lol!!!

Why not PM Durham Sparky here an ask him for his views on your problem/query!!

 

[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.