generator earth advise

[Farmelectrics]

in the building or one of them that we work in we have just had the building surveyed one of the faults that was high lighted was that the generator earth should be seperate from the building earth is this correct what we have is if the building looses power the ups kicks in allowing time for the generator to run essential services ie comms rooms so the bussiness can still operate the surveyer said that if we have a fault current it would take out the lv power and also the generator what he is saying is to prevent this the generator should be earthed to its own earth mat seperate from the buildings has anyone come across this or give us some advice on the subject

 

[PRR]

Not looking at regulations:

There's only one earth and we all share it.

The regs I know best say that Lightning Protection gets its own dirt-rod BUT that is bonded to the electric power dirt-rod and service line. In the case of an alternate power source for the same loads, I do not understand the surveyer's point.

 

[Engineer54]

in the building or one of them that we work in we have just had the building surveyed one of the faults that was high lighted was that the generator earth should be seperate from the building earth is this correct what we have is if the building looses power the ups kicks in allowing time for the generator to run essential services ie comms rooms so the bussiness can still operate the surveyer said that if we have a fault current it would take out the lv power and also the generator what he is saying is to prevent this the generator should be earthed to its own earth mat seperate from the buildings has anyone come across this or give us some advice on the subject

Ask this surveyer how you can isolate the building earth from the generator when it is supplying power to the same building?? You should however have the generators Neutral point earthed, via it's own ground rod(s)/mat.

 

[Farmelectrics]

i dont understand his point either prr another question for yourself or e54 ok are building has its own lighting protection and comes down to various rods in the ground so if the building is struck by lighting it travels down shortest route and disperses into the general mass of the ground why do we then link the lighting protection to the main earth bar surely were giving it a fault path back into the building your thoughts please

 

[lauriehurman]

Well I'm no expert so I may have misunderstood but what you have at the moment is the generator wired as an exposed conductive part - it is bonded to the main earth. When you have an earth fault the body of the generator goes to Line potential (230?) just at the time when you want the generator to start up.

I think (I may be wrong) that the point of giving the generator a "separate" earth is that planet earth is treated as an infinitely large sink so there's no chance of the body of the generator being at Line voltage during a fault.

I've no experience of generators so I may be talking rubbish

Laurie

 

[Farmelectrics]

thanks for your input laurie interesting were still trying to get our head round it ha ha

 

[Engineer54]

Well I'm no expert so I may have misunderstood but what you have at the moment is the generator wired as an exposed conductive part - it is bonded to the main earth. When you have an earth fault the body of the generator goes to Line potential (230?) just at the time when you want the generator to start up.

I think (I may be wrong) Laurie

You thought right, ...you are wrong!! lol!!!

The protection control system should not allow any change-over, when there is a fault existing on the system. Not sure what sort of protection you have on this stand-by gen-set of yours?

There is no bonding involved, only earthing. and seeing as there is no way of isolating the main systems earth from the generators earthing system, and little point even if you could.. I'm still trying to work out his fault current path, as the change-over switch should always be 4 pole on a 3 phase system and DP on a single phase system.

Any half decent mains/generator set-up would incorporate earth leakage protection anyway, where that EFP is located depends on the system hook-up etc...

 

[Farmelectrics]

so e54 in your experience are you saying the generator dosnt need to be on its own earth mat (earth) and could you explain to me why we take an earth from the met to the lightining conductor cos in my mind were bringing a fault back into the building why dont we let the lightining strike just travel down rods and disperse in the general mass of the ground why give it a path back into the building

 

[Knobhead]

The generator must have it’s own earthing provision. You can not rely on the DNO earth only. To do so puts anyone working on the distribution system in danger. They can be linked yes, but a separate earth must be provided.

 

[Rockingit]

My take on this has always been that you need the generator/s and the dno supply to be at zero potential to each other otherwise all sorts of strange and dangerous things could occur. You have a separate rod/mat/whatever for the generator which is derived from a neutral tap which then by default becomes a TT which is why most sets are then fitted with EFP of some flavour or another.

 

[Knobhead]

A correctly installed generator is TN-S

 

[Rockingit]

A correctly installed generator is TN-S

OK, I can see that - so do we just 'ignore' the addition of the rod?

 

[Engineer54]

so e54 in your experience are you saying the generator dosnt need to be on its own earth mat (earth) and could you explain to me why we take an earth from the met to the lightining conductor cos in my mind were bringing a fault back into the building why dont we let the lightining strike just travel down rods and disperse in the general mass of the ground*** why give it a path back into the building

As stated previously, you need to provide an earth rod/mat connection to the generators N-E point/position, which will effectively earth the generators frame as well. And '''NOT'' a 200 ohm affair either!! lol!!

***If only lightning strikes always followed those made paths, and ONLY those paths to ground.. lol!! In reality lightning strikes don't always follow those made paths and can find/seek out other paths, by means of jumping or penetrating...


As for lightning protection, one reason is that the LP system will introduce an extraneous earth situation, say on the roof of a building where different services are present (A/C, Water Tanks, Aerial Installations etc. So to bring the LP to the same potential as the main supply earth, a link or bond is made, normally to the LP down conductor closest to the buildings main intake.

Another reason is that most buildings that support an LT system will be of reinforced concrete construction, (some systems will even use the reinforcement as part of LP the down conductor). It is more than likely that any decent strike on such a building, will reach any reinforcement (normally only around 25mm inside the concrete) anyway.

As this reinforcement will at some point or points be connected to the buildings power supply earthing arrangement, whether it be by a purposely made connection or otherwise it is again sensible to link the LP to the building earthing system. Last but certainly not least, it will effectively stop any jumping of lightning strikes from the LP system to any building earthed metalwork etc!!

The use of SPD's on electrical installations where LP systems are in place, will effectively reduce or even completely remove any lightning strike damage to the system... To be honest, i've never seen any damage caused by lightning strikes to LP protected buildings, that have been totally devoid of such devices, but i would now never not use them, as building system sophistication is increasing exponentially!! lol!!

 

[Knobhead]

In reply to Rock,

No you don’t ignore the rod or mat, it is the origin of an independent TN-S system. Just like the DNO the earths are linked but unlike the DNO systems the neutral can’t be linked, hence the need for a 2 pole centre off change over switch.

This is all to protect anyone working on the distribution system.

 

[Clive P]

Hi,

I've already put my tin hat on! I have always been told and followed the instruction that Lightning protection systems should be totally independent of Buildings or sites Earthing arrangements. I agree with the use of SPD's but this can be complicated and expensive.
To be honest a lot of the work I have done has been Hazardous Areas and the regulations may be interpreted differently,

Regards.

 

[Engineer54]

Hi,

I've already put my tin hat on! I have always been told and followed the instruction that Lightning protection systems should be totally independent of Buildings or sites Earthing arrangements. I agree with the use of SPD's but this can be complicated and expensive.
To be honest a lot of the work I have done has been Hazardous Areas and the regulations may be interpreted differently,

Regards.

Well whoever told you not to link the LP system to the buildings MET probably knew little about the workings of an LP system. Now i'm not going to say i know enough about LP systems within designated Hazardous area's, but i would think that it was even more prudent to include that link to the buildings MET for the very reasons outlined above. Unless someone here can give solid reasons why such a system wouldn't/shouldn't be linked...

 

[MDJ]

No you do connect a drop to earth, on every lightning conductor I have been responsible for the company I employed to install the system always made me connect to it.

 

[Farmelectrics]

thanks for all your input people a bit clearer but not totally lol

 

[Billythekid]

Ensure Generator does indeed have an Earth mat/nest, and fit new MET with removable links?

This is what i've always done, and although fitted i've yet to see a link removed whilst on gen set. This is actually written in the emergency plans/method statements, you can take them to the water . . .

Regards

Billy

 

[PRR]

Re: separate earthing for generator: Tony's right. In many places "earth" comes from the Power Utility Company as well as possibly dirt-rods/mats. In a power failure, the utility ground is not trustworthy. The line from the street may be broken. Or it may have to be cut to perform repairs. The generator needs an earth which will function even when utility power breaks.

Whether this be a dedicated rod/mat or the normal building ground depends a LOT on local regulations and power source type TN (TN-C TN-S TN-C-S) TT or IT, the required or orderly sequencing of PEN MET and PME terminals and service jumpers.

Residual-Current breakers may reduce (hardly eliminate) shock hazard, but that's not the same as grounding.

___________________________________________
> building has its own lighting protection and comes down to various rods in the ground so if the building is struck by lighting it travels down shortest route and disperses into the general mass of the ground why do we then link the lighting protection to the main earth bar surely were giving it a fault path back into the building

Follow the trails.

Power electricity flows only from brown to blue and back. Ultimately all the way back to the generator.

Lightning flows cloud to earth (or earth to cloud, same as far as we care).

So it is really two different paths. Like a highway and a railroad. We don't want the cars to end up in the railway yard, nor locomotives in the shopping center, even though both are "on earth".

If we had cheap perfect insulation, we could keep the two flows apart.

But lightning strikes can be a million volts. We can't insulate for that. And utility power systems run both near dirt and to top of buildings and towers. And they cover vast distances. When there is lightning around, odds are that a good percent of strikes will go "through" power wires on their way from cloud to earth.

A lightning bolt can be VERY high current, many-many thousand Amps. Even where it enters earth, dirt resistance means the voltage at the surface will be elevated from "normal" voltage.

Power utility insulation breaks down. At 10,000V you have sparks jumping out of wall sockets and jumping through brown blue and green plastic. At 100,000V the huge distribution transformers will break-down. Somewhere before that, arc-horns and other protectors will be trying to divert the potential.

It's a real mess. And if Thor is going to get you, he WILL. But he often near-misses.

There is no good answer. Except to tear-out all our puny wires, and the risk of lightning is low and the benefits of power are large, so we won't do that.

The power company "must" bond their lines and gear to local dirt throughout the system, so that lighting strikes have a short direct path to earth, with massive redundancy so that if one or a dozen dirt-rods are blown from the earth, others further down the street will take the overflow so that the entire system does not domino and repairs will be limited.

Imagine a hit on your lightning-rod going to its dirt-rod. The high energy "lifts" the earth potential many thousand volts. Meanwhile your power dirt-rod a few feet over is still at "normal" potential. Electricity is sure to flow, and violently.

Now bond the two rods together. Your electric wiring is "lifted" part-way toward the lighting-induced dirt potential, reducing the tendency to flow, and it flows in a designed conductor instead of blasting dirt and jumping through walls. Yes, this means your house wiring potential is now thousands of volts higher than the dirt far from your house or the power company wire along the street. There will be surge and possible damage. But the inside of your house is (probably!!) safer than if lightning and power dirt-rods were not bonded.

Other things can happen. Lightning is not easily predicted. The exact course and energy can't be known even at the moment, much less decades before when house and power and rods are installed. And different places vary. I'm on dry rock. I can't get a good dirt-bond. I might be safer with oil-lamps and no power line. (But oil-lamps caused many tragic fires....) Power companies treat dirt-bonding in different ways, and also treat customers in different ways (some expect to "give" earth to customers, others prefer customers find their own path to earth). Even when lighting and power are bonded, the bond conductors, rod spacing, resistance goals, and other details vary with local experience and conditions.

So best left to experts who are insured or can get you better insurance. However when they drive separate rods and then tie them together, it is pretty reasonable and widely accepted.

 

[i=p/u]

mwah xx