generators

[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!!

 

[sparkx]

This is a boat and is outside the scope of BS7671 - think you need to consult a grown up.

As a previous poster has mentioned you need to have fault monitoring devices installed.

 

[Knobhead]

Until we know the precise method of “earthing” the generator neutral it’s like trying to plat fog.

Totally impossible.

You keep prevaricating around the questions, which leads me to think you’re in the mire over your head.

I’ll PM Durham

 

[DurhamSparky]

hehehe!! im in no position to give advice to the un advisable!

on a water based craft we had NO earth and tbh EFLI and RCD's we unheard of in the RN..!
yes we had a constant IR monitoring system checking for Earth Faults to the Hull of the ship..! our limit was under 600k ohms then we had to investigate, on a good day 999m ohms was achievable but seen as our MG sets used to eat carbon brushes for dinner it soon dropped rapidly! out come the Air guns and rags!! on completion it seemed like scene from marry poppins and the chimney sweeps..!


any way back to the point...! do you have a Neutral Earth TX at the generator? to assist with fault current when starting up and running etc..?

 

[Marvo]

You need a marine engineering consultant, this area is a minefield and there's many variables you need to consider before designing this kind of system. The system needs to consider safety of people on the barge as well as people not on the barge primarily but corrosion implications can cost astronomical amounts if not considered correctly.

I'm not an expert in this field but the marine projects I have been involved in have taught me you can't cut corners.

 

[DurhamSparky]

the RN cut corners!!???

 

[general error]

Hi again Robbie

The generator is likely to lose output voltage after half acycle when delivering fault current. personally would give greaterconsideration to this then I would to Zs’s.
Also. Residual current devices may become unreliable in saltladen air.
If you are going to get the blame for something badhappening it’s likely to be in connection with a job like this one so doconsider all the “what if’”s you can think of . first. Good luck.

 

[Marvo]

Also. Residual current devices may become unreliable in saltladen air.

I was assuming 'barge sits in the middle of a loch' meant freshwater.

 

[general error]

I was assuming 'barge sits in the middle of a loch' meant freshwater.

Probably but there are a few salt water inlets also named loch,Please would Robbie dip his finger in (with the generator locked off) & report back

 

[Marvo]

Probably but there are a few salt water inlets also named loch,Please would Robbie dip his finger in (with the generator locked off) & report back

Lol, the swearword filter doesn't like you talking about salt-water.

I'll stand to be corrected but it would certainly influence the installation method. Some more info on the generators would also be nice. This is still a job I would refer to someone more qualified for a spec to be issued.