Connection Of Generator

[gselectrical]

Evening all. 

We have a client who would like to connect a 16kVA generator to his property during mains failure.

The generator will be housed in a nearby field and connected when a power cut occurs. So no need for it to kick in automatically upon power failure. 

We plan to install a 63A single phase industrial plug for him to connect the generator along with a change over switch. 

My question is, how should we isolate the DNO's earth when the generator is required? Is it a case of wiring this through the changeover switch?

Cheers

GS

 

[steptoe]

How are you earthing the genny,?

Is it a star point earthed genny,?

Need a LOT more info.

Oh, and yes, 

I really don't think you should be using the DNO earth, how might this powercut occur,? Broken cable,??, 

 

[Andy™]

your genny will need an earth rod. but you may not need to disconnect DNO earth

 

[Evans Electric]

You must take heed of not back feeding down the DNO supply during the power outage for a start.

 

[gselectrical]

Thanks for the reply's guys. Much appreciated as always. 

My clients have a severely disabled son, who needs 24/7 care and power is a must. They have had 6 power cuts this year alone. (Remote location)

The earthing system is TNC-S. 

The building has its own transformer on a pole approximately 125m from the property. 

We intend to install an earth rod for the genny. 

I know from previous experience at this site that we will not be able to achieve <20ohms on the rod, which is why I am asking about disconnecting the TNC-S system when the gennie is required.

I am assuming we won't back feed down the DNO's supply during outage as we intend to install a change over switch?

Cheers

GS

 

[steptoe]

why would you need <20 ohms ?

that is very VERY low for a rod,,,,,,,,,

 

[Manator]

probably a mistype for 200 Ohms

 

[Phoenix]

why would you need <20 ohms ?

that is very VERY low for a rod,,,,,,,,,

Thats what BS7430 specifies when the earth electrode is used to tie down the neutral point of a generating set:

Generating sets may be earthed to limit the potential of line conductors withrespect to the general mass of Earth particularly under fault or unbalanced

conditions, and earthing is generally necessary as part of the protection against

electric shock (see Figure 10 to Figure 12).

Generating sets should be protectively earthed by connecting the frame and the

neutral point of the generating set, associated exposed-conductive-parts and

extraneous-conductive-parts to a main earthing terminal. The earthing terminal

or bar should be connected to an independent earth electrode. The earth loop

impedance at any point of the installation should be low enough to ensure

operation of the fault protection, and this should be taken into account if the

earth electrode forms part of the earth fault loop, such as for a TT system (but

not TN).

For independent earth electrodes associated with the local earthing of the star

point of generating plant, the earth resistance should not exceed 20 Ω.

 

[Manator]

Thats what BS7430 specifies when the earth electrode is used to tie down the neutral point of a generating set:

It is not a Ra reading though is it?

 

[gselectrical]

why would you need <20 ohms ?

that is very VERY low for a rod,,,,,,,,,

As this is what the DNO require for a rod to be connected to their earthing system (PME)?

Cheers

GS

 

[steptoe]

As this is what the DNO require for a rod to be connected to their earthing system (PME)?

Cheers

GS

oh dear oh dear

NO, you have absolutely nothing to do with what the DNO supply on a PME system,

I thought you said it was TNCS anyway?

why would you be concerned about what the DNO require THEIR rods to be?

2nd, you CANNOT connect your genny in any form whatsoever to the DNO network, absolutely NOT,

unless of course, as has been mentioned previously, you conform to G38 or G59 ,  which is sounding more and more like a definite no.

It is not a Ra reading though is it?

not the way Im reading the article

earth loop

impedance

 

[gselectrical]

Sorry for the late reply, late day at work yesterday!

Thanks for the reply steps.

oh dear oh dear ?

NO, you have absolutely nothing to do with what the DNO supply on a PME system, Ok, thaught so.

I thought you said it was TNCS anyway? Apologies, I should have been more clear. This is a PME system.

why would you be concerned about what the DNO require THEIR rods to be? I am not, I am converned about what our rod should be.

2nd, you CANNOT connect your genny in any form whatsoever to the DNO network, absolutely NOT, Is this regarding the earth? I am aware Line & Neutral can not be connected to the network in any way, which is why we intend to install a change over switch. My concern is separating the 2 earthing systems IF we need to. 

unless of course, as has been mentioned previously, you conform to G38 or G59 ,  which is sounding more and more like a definite no.

not the way I'm reading the article
 

 

[gselectrical]

Hi Guys. 

Does anybody have any suggestions on the best way to separate the earthing of the generator and the DNO's earth when the generator is required?

Cheers

GS