Generator

[steptoe]

Guys this is getting heated, i can appreciate why but lets cool it.We are now moving into FMEA's Failure Modes & Effects Analysis.

We have an engineered system.

When operating as designed there is no problem.

Steps is suggesting, sorry Steps, I have just realised that i did not capitalise my previous posts correctly.

Steps is suggesting that in a direct contact failure mode the effect on the system is to deliver the fault current via the same modus operandi of a TT system.

Steps?

yes,

regardless of the supply method,

under DIRECT fault conditions then it undertakes the same principles as TT

that is why it is imperative the genny/star point is earthed.

this is also(IMO ) the same reason a DNO supply is spiked/rodded at star point regardless of the distribution method.

EDIT,

this is how I understand the method of operation.

and that is what I was trying to explain/suggest,

rather badly obviously.

 

[steptoe]

So if the orginal source of supply was TT, then its still TT, unless we supply an earth from the Genny, or if it was tnc (-s) its still that if we supply the earthonly difference is, is our new earth as good as the old one?

we cannot supply TNCS, not from a standard genny at least, our supply from a genny is very similar to a sub main,(if we can be simplified here).

in that we will probably have protective devices and RCDs readily available to reset, and the earth will be available as a separate terminal.

 

[steptoe]

Can we agre the Genny need to be earhthed at the star point ?

yes, I would say so.

it should be,

some posters are trying to say it is optional.

that is the issue.

they dont understand earthing procedures.

 

[philbas]

Im trying to look at thist from a simple neewbie point of view>

the Genny takes over form the DNO.

Been realistic you cant do tn-c-s

no space to split

so its either tt or tn-s

now we have to ask what was it to start with, if it wassTT then the odds are the earth from the genny is not going to be as good as the original TT earth, if it was TN of some sort then we lost that when the DNO pulled the plug, so now we need to go toTT, you cant depend on the rod from the genny to jou need to rod the installation. am I wrong ? I do want to learn

 

[Doc Hudson]

Following complaints, I have read through all of this thread. I will not comment on either side of the debate, But it has been sailing a little close to the wind. I ask ALL members to please keep their manners and politeness settings on high.

I thank ALL members for their input, broad context of input and debate is one of the positive points of this forum, without it, it becomes stale. We all need to check on why we hold a particular point of view, and to debate issues in a constructive manner.

I believe there is no other UK forum that allows the level of polite debate that has been achieved on this forum. So please gents, take a breather, and adjust your posts if you find someone else's post is getting you a bit to hot under the collar.

Thank you for your help keeping the forum running smoothly.

Doc H.

 

[steptoe]

Im trying to look at thist from a simple neewbie point of view>the Genny takes over form the DNO.

Been realistic you cant do tn-c-s

no space to split

so its either tt or tn-s

now we have to ask what was it to start with, if it wassTT then the odds are the earth from the genny is not going to be as good as the original TT earth, if it was TN of some sort then we lost that when the DNO pulled the plug, so now we need to go toTT, you cant depend on the rod from the genny to jou need to rod the installation. am I wrong ? I do want to learn

if it was originally TT then we still have the option to use the genny supplied earth,

it is in "effect" a TNS supply, but from a genny (and I simplify here) it arrives to us as a normal submain would in the form of LNE.

now we can use this LNE supply as we would a normal submain and just plug into it with no abnormal effects, and all is well.

BUT

the earth from the genny is an artificial earth, and is not the same as the "general mass of earth",

at this point we now have the genny producing a T it also produces an N as well, we just have to decide how the genny will deliver these.

the delivery of L is already decided, we are not allowed to change that method, as is the same with N, we can only decide if earth T is delivered Separate, or Terra.

 

[steptoe]

am I describing this rubbish?

 

[philbas]

Im not been awkward I really do want to know

 

[steptoe]

Im not been awkward I really do want to know

so, following on, we now have a L & N at our point of contact,

we now need an earth T ,

how do we get it there,

we can run another cable, sometimes the most simple method, Separately from L & N , but what if it gets broken, then we wont have an earth,

or even if it doesnt get broken, what do we do in case of a direct fault,

well we could rod the star point anyway just as a back up,

so if we are going to rod the star point as a back up, why dont we do away with the extra cable and just rod the MET too, then we wont have a cable to drop, mand if the N drops then at least T doesnt go live, and if L drops then no potential to worry about.

 

[Sidewinder]

Could not carry on the debate last night was tooooooo tired!

I think now Steps I'm beginning to understand your thought processes!

This is one of the problems with this method of communications it can be too literal and difficult to fill in the blanks between the words which the tone of a voice and body language can sometimes help with.

I believe that the star point of the genny should be earthed.

Below a certain size for single tools etc. other systems can be devised by the manufacturer of the device inbuilt which provide adequate protection.

We won't go down that road as the genny here is large.

I don't see we have any choice for the supply other than TT or TN-S

TN-C-S is the preserve of the DNO territory as CNE conductors are prohibited except for use by DNO's

So as Steps says in his last post the delivery of L & N is pretty fixed.

The ONLY question is, how do we earth the install and the generator and connect these?

Generator star point earthed, this ties the N to earth at the genny, ala TN

Agreed? End of, this must be done.

So as Steps says the only question is how we link the earthed star point of the genny and the MET of the installation.

Options

1. Do we go S i.e. provide a separate earth conductor from install MET to genny star point. TN-S

Or

2. do we use the general mass of earth to link the two? TT

Or

3. do we TT and then link with a separate earth conductor? PME "type"

Conclusions:

1. As Steps says, if the S earth conductor is compromised the install is without earth. This would be true of any TN-S system, it is dependent upon the integrity of the earth "wire" supplied to provide the earth. How sure can you be of the integrity of this conductor?

2. Probably the highest earth value, but does not rely on anything to conduct any earth current except the general mass of earth. Nothing to fail barring an earthquake!

3. probably a lower overall earth resistance due to the multiple earths should the connecting wire fail, the system would default to TT. Probably the best option? However if you can't ensure the integrity of the "earth wire link" then you should provide protective devices for TT. That is design as TT and add the link of a suitable size as a bonus to ensure that the two "earths" remain at the same potential as far as possible.

BTW HSE publish some guidance, this below amongst others.

http://www.hse.gov.uk/foi/internalops/fod/oc/400-499/482_2.pdf

it is a little old, but still valid for HSE Field Operations Directorate (field enforcement officers)

Going out for a while shortly will pick this up later.

 

[Mr Smith]

as with my colleagues above

i have read through the whole topic from a laymans perspective

following a complaint

my concern is members will be afraid to post questions

only to see their original simple query get swallowed up in some heated debate that is almost off topic

thanks are due to mr winder for his articulate and informative posts which appear to have defused the situation

other than thati have nothing else to say that mssrs hudson tor and dog have not said before

mr smith

 

[Sidewinder]

I have just re-read some of the early posts.

Colloquialisms included in an attempt to keep the description clear

.

I THINK that the confusion / argument has come from terminology related to TT/TN-S and the disconnection of the DNO supply and the replacement of this with the genset.

The DNO supply MUST be completely disconnected all 3 wires.

You are replacing the DNO supply with the genset.

Your earthing for the new supply must not rely in any shape or form on the old supply.

Think if your DNO provided a new supply head, you would replace the 3 wires from the old head with the 3 for the new head.

Allowing for LNE connections provided by DNO.

Thus you have to rod the genset star point/case, thus T.

I think my last post describes the remainder of the situation.

One last comment though. You COULD be cheeky and connect to the DNO earth, not correct but it would pull down you Zs's by reducing "Ze". Probably not allowed by ESQCR?

Finally I've not thought through the whole ramifications of connecting the genset/installation earth to the national grid earth provided by the DNO.

That should perhaps have some serious FMEA before it is done.

 

[a1]

These are 3 of the best posts and clearly explain it.

if it was originally TT then we still have the option to use the genny supplied earth,it is in "effect" a TNS supply, but from a genny (and I simplify here) it arrives to us as a normal submain would in the form of LNE.

now we can use this LNE supply as we would a normal submain and just plug into it with no abnormal effects, and all is well.

BUT

the earth from the genny is an artificial earth, and is not the same as the "general mass of earth",

at this point we now have the genny producing a T it also produces an N as well, we just have to decide how the genny will deliver these.

the delivery of L is already decided, we are not allowed to change that method, as is the same with N, we can only decide if earth T is delivered Separate, or Terra.

so, following on, we now have a L & N at our point of contact,we now need an earth T ,

how do we get it there,

we can run another cable, sometimes the most simple method, Separately from L & N , but what if it gets broken, then we wont have an earth,

or even if it doesnt get broken, what do we do in case of a direct fault,

well we could rod the star point anyway just as a back up,

so if we are going to rod the star point as a back up, why dont we do away with the extra cable and just rod the MET too, then we wont have a cable to drop, mand if the N drops then at least T doesnt go live, and if L drops then no potential to worry about.

Could not carry on the debate last night was tooooooo tired!I think now Steps I'm beginning to understand your thought processes!

This is one of the problems with this method of communications it can be too literal and difficult to fill in the blanks between the words which the tone of a voice and body language can sometimes help with.

I believe that the star point of the genny should be earthed.

Below a certain size for single tools etc. other systems can be devised by the manufacturer of the device inbuilt which provide adequate protection.

We won't go down that road as the genny here is large.

I don't see we have any choice for the supply other than TT or TN-S

TN-C-S is the preserve of the DNO territory as CNE conductors are prohibited except for use by DNO's

So as Steps says in his last post the delivery of L & N is pretty fixed.

The ONLY question is, how do we earth the install and the generator and connect these?

Generator star point earthed, this ties the N to earth at the genny, ala TN

Agreed? End of, this must be done.

So as Steps says the only question is how we link the earthed star point of the genny and the MET of the installation.

Options

1. Do we go S i.e. provide a separate earth conductor from install MET to genny star point. TN-S

Or

2. do we use the general mass of earth to link the two? TT

Or

3. do we TT and then link with a separate earth conductor? PME "type"

Conclusions:

1. As Steps says, if the S earth conductor is compromised the install is without earth. This would be true of any TN-S system, it is dependent upon the integrity of the earth "wire" supplied to provide the earth. How sure can you be of the integrity of this conductor?

2. Probably the highest earth value, but does not rely on anything to conduct any earth current except the general mass of earth. Nothing to fail barring an earthquake!

3. probably a lower overall earth resistance due to the multiple earths should the connecting wire fail, the system would default to TT. Probably the best option? However if you can't ensure the integrity of the "earth wire link" then you should provide protective devices for TT. That is design as TT and add the link of a suitable size as a bonus to ensure that the two "earths" remain at the same potential as far as possible.

BTW HSE publish some guidance, this below amongst others.

http://www.hse.gov.uk/foi/internalops/fod/oc/400-499/482_2.pdf

it is a little old, but still valid for HSE Field Operations Directorate (field enforcement officers)

Going out for a while shortly will pick this up later.

as with my colleagues abovei have read through the whole topic from a laymans perspective

following a complaint

my concern is members will be afraid to post questions

only to see their original simple query get swallowed up in some heated debate that is almost off topic

thanks are due to mr winder for his articulate and informative posts which appear to have defused the situation

other than thati have nothing else to say that mssrs hudson tor and dog have not said before

mr smith

100% Agree with all Mods.

Healthy Debates - YES.

Name calling etc. NO!

 

[M107]

I've only read the last few posts & have to say thanks to steps & sidewinder...

Fantastic explanation.

 

[Manator]

We can all see how difficult it is to express an idea through the written word. Both Sidewinder and Steptoe have done an excellent job in keeping the debate about the subject and not about the personalities.

The posted information provided by all contributors to this debate will prove to be very helpfull, and will give thoughts to all those who may have to install a generator in the future.

I would like to thank all the posters in this thread for a very informative and in the end very balanced debate.

 

[spinlondon]

One of the problems with this thread, is the refernce to the star point of the generator.

The installation as far as I can ascertain is single phase, and I assume that so will be the generator. As such there will be no star point.

There is no requirement in BS7671, or BS7430 (Earthing Code of Practice), to use an independant earth. In fact it is accepted that these generators can be run quite satisfactorily as floating systems.

If an RCD is required (usually where there is likelyhood of damage to supply cables), or the installation has existing RCD protection. An independant earth must be used, and a connection made between earth and an unswitched pole (often this connection is done in the supply cable) this unswitched pole then becomes the neutral.

The independant earth can be placed anywhere between the generator and the RCD, it is recomended that the impedance of the rod be no greater than 20ohms.

 

[batty]

Next weekend have got to install some sockets for marquee at charity I do work for for there open day these are being run from generator that is being hired, apparently it has it own built in RCD so will it need rodding.

 

[kme]

Generally, it is recommended to rod `em locally, and attach to chassis, UNLESS the manufacturers data says different.......

KME

 

[spinlondon]

Next weekend have got to install some sockets for marquee at charity I do work for for there open day these are being run from generator that is being hired, apparently it has it own built in RCD so will it need rodding.

Here's a link to an article in the IET wiring matters.

http://www.theiet.org/publishing/wiring-regulations/mag/2006/20-entertainment.cfm?type=pdf

It's a bit out of date, as it makes reference to 16th Regulations.

 

[batty]

Bringing back an old thread generator is on site. It is supplied with its own rod which has already been installed buy supplier. Doing a loop test you get same readings l-n as l-e so assume its TNS so rod is only a safety precaution.