generator earthing 1

[keewee]

Hi I live in New Zealand, I'm new to "eng-tips" and I have a sticky question:

If a hospital main switchboard has an emergency generator transfer switch. Should the transfer switch break the Neutral on the generator side ? And what should the earthing/grounding arrangements for the generator and the main switchboard be ?

What are the implications of a "seperately derived" emergency generation system for a medical installation ?

In NZ we have a Muliple Earthed Neutral System and if anyone has any experience with generator ATS systems in the MEN system I would appreciate any input.

 

[dpc]

I think the answer is going to be highly specific to your location, local codes, ground fault relaying, and grounding methods.

I wouldn't even venture a guess for New Zealand. But we have a lot of folks from the UK so maybe their experience will be more relevant.

 

[WattoUK]

Hi keewee.

In the UK, standby generator installations that operate as a switched alternative to the normal utility supply (ie the genset is not designed to synchronise and run in parallel with the utility network) only require to use 3-pole switching for PME (Protective Multiple Earthing) supplies at low-voltage. In other words, I think your local NZ code will not require the generator neutral to be switched.

As far as earthing the standby genset goes, again, in the UK the generator neutral starpoint must be efficiently earthed using an independent, measured earth electrode. The electrode impedance must be less than 1 ohm and is typically less than 0.2 ohm. The independent earth electrode must also be solidly bonded to the earth (or neutral/earth for PME) electrode supplied by the utility company (assuming they do supply one!).

As I am a utility engineer myself, I do not have any particualr insight into the specific requirements for medical installations. However, it is worth keeping in mind for any standby generator installation that the available fault level (or PSCC, if you wish) is likely to drop significantly while you are running on your generator.

The main implications for this are (1) any plant that has a significant starting current, such as pumps or lift motors will cause more severe voltage dips - this may have an unacceptable impact on the operation of certain medical equipment. (2) lower fault level will cause increased harmonic distortion of the voltage waveform by non-linear loads - under certain (albeit fairly severe) circumstances, the increased HD can cause UPS units to reject their loads. I would imagine this would be particularly undesirable in a hospital setting!

Your local supply utility should be able to advise you on all the points I have mentioned.

cheers
Watto

 

[ScottyUK]

Watto,

Can I add a third item to the effects of reduced fault current: clearance times of breakers may be increased, particularly on larger breakers which are a significant fraction of the set's rating.


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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[WattoUK]

Scotty -

Yes, of course, that is a very significant factor to take into consideration. In my limited experience it is also one that is rarely incorporated into the design of electrical installations where the genset is purely for standby power.

Thanks for keeping me/us right!

cheers

Watto

 

[keewee]

Thanks for your replies guys.

WattoUK. If the emergency generator neutral and earth are connected to the Main Switchboard Earth. Could this pose a problem if the generator is started ( maintenance test ) while the mains supply is still available. If the generator neutral isn't switched and the generator isn't perfectly balanced, could this result in a rise in potential in the switchboard Earth duing a routine generator test ?

Sorry if this sounds like a load of blarney but what I'd really like is some drawings or references to codes that outline the earthing requirements in an emergency generator changeover switchboard for a medical installation.

 

[WattoUK]

keewee -

Before I go any further, I just have to remind readers that I am approaching this subject from a UK regulatory / statutory perspective.

In the UK it is illegal to operate any electrical distribution network (of any size) without it being efficiently earthed.

A standby generator installation requires to have, amongst other things, a measured independent earth electrode so that should the utility supply network 'disappear' (eg overhead networks flattened by storm-force winds), the standby genset can still supply the electrical installation AND be referenced to the general mass of earth.

Now, if the utility supply is of the separate neutral/earth variety (neutral only earthed at ONE point, back at the local substation - that's TN-S to non-utility people), then you are quite correct; the neutral is required to be switched too using 4-pole switchgear.

If, however, the supply is from a network where the neutral and earth conductors are combined, as in protective multiple earthing (PME, or TN-C-S, if you prefer), then the genset changeover switchgear is only required to be 3-pole. 4-pole is acceptable, but it is not a requirement. The neutral/earth conductor from the utility is efficiently earthed at least at one point (the substation) and most probably at many other points too, just like your multiply earthed neutral network.

Now, regardless of which flavour of utility supply you have, if the utility provides you with an main supply earth terminal, you are required to bond it solidly to your independent earth electrode, and hence to the generator starpoint. In the UK some LV supplies such as petrol filling stations are not afforded an earth terminal.

If you have a PME or MEN supply, the neutral of the utility supply is also directly bonded to the generator neutral as a consequence of the network earthing arrangement - so only 3-pole switching is required. If you elect to utilise a 4-pole switch, the generator neutral/earth is bonded via the main earth terminal to the utility neutral/earth anyway, so why bother?

I accept what you say about unbalanced loading of the genset 'imposing' a rise of potential in the overall earthing but as everything is bonded together and the prospective potential gradients are small, there are no significant step, touch or transfer potential issues.

Any utility work on the neutral conductor, cable sheath or substation earth electrodes already takes into account the risk of transfer potential due to unbalanced generators, poorly wired (reverse polarity) installations and genuine earth faults as part of published live working procedures.

FYI, the statutory basis for electrical installations in the UK is the Electricity Safety, Quality and Continuity Regulations 2002, which may be viewed at-

http://www.dti.gov.uk/electricity-regulations/index.htm

The industry standard for interfacing utility supplies to generator installations is Engineering Recommendation G59/1, published by the technical documentation wing of EA Technology (

http://www.eatechnology.com).

However, the document I think you are seeking is the sister document to G59/1, which rejoices in the equally forgettable title of Engineering Technical Report 113 (or simply ETR113). It is published by the same body and contains the technical treatment with diagrams that you seek. It is VERY pricey though, coming in at over 200 UKP per copy.

Finally, in the UK, fixed electrical installations at LV are regulated by the current edition of the IEE Regulations for Electrical Installations, commonly known as the Wiring Regs and less commonly known as British Standard 7671. Specific issues related to generators are addressed in this document too.

As I said in my first reply, your initial point of reference should be the operations engineering department of your supply utility. An engineer from that department would/should be able to give you all the information you need.

regards

Watto

 

[keewee]

Watto thanks for your advice.

 

[PowerfulStuff]

keewee,
Jumping across the Tasman Sea to Australia, some power distribution companies have it in their contracts that they must approve any standby generator connections. The distributors can be quite helpful is providing information as to what is required to the Australian/NZ standards and their own requirements.

Regards,
PowerfulStuff

 

[keewee]

Thats right Powerfulstuff, I never thought of that. If the Power Distribution Company certifies the supply to the transfer switch then they must have line diagrams of the Earth & Neutral arrangements they require. I'll try get my hands on those drawings then. I'll let you know how it goes.