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Standby generator eart connection?
3

Standby generator eart connection?

Standby generator eart connection?

(OP)
Advice required on the connection of a mobile standby generator.
We have a sub station with a LV Earth to which the star point of the supply transformer is connected. The supply to is 415V 3-phase + Neutral. Also connected to the LV earth is the earth bars & metalwork of the LV switchgear. We have a separate earth for the HV switchgear metalwork.
We have a connection point for connecting a standby mobile generator when required, and this connects to our LV board and supplies power by way of a changeover switch. This is interlocked with the mains supply so that only one source can be on at any one time.
Now we have had a visit by some copper thieves and all the earthing has been nicked. I am in the process of installing the earths again, but not quite sure what to do with the generator earth.
I thing I'm right in saying that a standby generator should have its own independant earth, but I am concerned that this will get nicked again. What I'm thinking of doing is running an earth connection from the generator point to the LV earth inside the sub. If I need to connect a generator to supply standby power if mains power is off, then the generator earth would be connected to the LV earth, and I would remove the link connecting the transformer neutral point to the LV earth. Replacing this link and disconnecting the generator when mains is to be restored.
Is this OK?

TIA

RE: Standby generator eart connection?

This type of portable generator connection is common is telecommunication facilities. The portable generator connection point should have an isolated neutral bus which is cabled or bussed to the main neutral bus, which is grounded or earthed at that point.  The portable generator connection point should also have a ground bus which is connected by equipment grounding conductor(s) to the ground bus in the main service equipment.  When you connect a portable generator, the generator neutral is grounded or earthed through the connection from the neutral bus in your generator connection point.  You then connect a properly sized grounding conductor from the frame of the generator to the ground bus at the generator connection point. The generator connection point should be a lockable cabinet with the phase,neutral and ground connection points (usually sections of bus bar).  It is also a good idea to label the interior of this potable generator connection cabinet with the proper system rotation.  If done this way there should be nothing exposed to get nicked.

RE: Standby generator eart connection?

(OP)
And would you remove the link on the LV earth bus to isolate the transformer neutral earth when running the sire by generator only?

RE: Standby generator eart connection?

If the substation is your own then you can utilise the substation LV earth. If it belongs to the distribution Company then you should seek agreement to utilise it. This agreement would not be unreasonably witheld, and it is a common arrangement. There is no need to remove the transformer neutral earth connection, and if you do you run the risk of someone forgetting to replace it, leaving the system unearthed which could be potentially very dangerous.
Regards
Marmite

RE: Standby generator eart connection?

To summarize the advice.
1. Use a grounding conductor to ground the generator frame to the main ground grid.
2. Connect the generator neutral to the system neutral so as to use the existing system ground to ground the generator neutral.
3. A separate ground system for the generator is a code violation in many areas. It may lead to lethal touch and step voltages in the event of a fault.
4. A three pole transfer switch with a solid neutral connection is the simplest, most dependable connection. A four pole transfer switch may be needed for some types of ground fault sensing schemes, but many four pole transfer switches are installed where they are not needed through incomplete understanding of the system.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Standby generator eart connection?

(OP)
Thanks for all the help.

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