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Topic Title: Earthing & Fault Protection of Vehicle
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Created On: 25 February 2013 10:42 PM
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 25 February 2013 10:42 PM
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kevincullen900

Posts: 7
Joined: 25 July 2008

Hello All.

I have read through a few similar topics, but I would appreciate some advice on the earthing requirements and fault protection on a transportable vehicle.

This vehicle (essentially a lorry chassis) contains a 20kVA engine driven generator, 3Phase 400V at 50Hz. This powers only equipment on the vehicle. There is however a 4 pole manual changeover switch to select between the on-board generator power and an external power supply via an inlet.

Earthing of this vehicle via an electrode etc will not be possible due to the locations it which it will be utilisied.

I was hoping for some advice as to the best method of earthing and fault protection. From what I have read so what seems to come up mostly is to make the generator a TN-S system by bonding the generator star point to earth, the frame f the generator etc.
The output would then be protected with an MCB and an RCD.

Initially my thoughts were to fit an insulation monitor to the installation and have this trip out the generator and additionally stop the prime mover, an IT system on the generator side.

I was wondering if anyone had ever come across a similar installation and offer some advice as to which solution would be most suitable, bearing in mind the possibility of an external supply.

Thanks in advance.
 26 February 2013 09:52 AM
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AJJewsbury

Posts: 10964
Joined: 13 August 2003

Sounds like section 717 of BS 7671 applies in this case - probably worth a bit of study. The lack of a true earth for the local generator shouldn't be an issue, although it class 2 protection around the generator up to the RCDs seems to be a common theme. I didn't notice any requirement to halt the prime mover, other than in the case of the vehicle being involved in a collision.

- Andy.
 27 February 2013 06:46 PM
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kevincullen900

Posts: 7
Joined: 25 July 2008

Thank you Andy

Just thinking as BSEN60204 is stated as being exempted from this section as "mobile machinery".
It however doesn't provide quite the same detail as section 717.

From my understanding of both documents it wold be possible to use the machine as an IT or almost TN-S.
I am not sure which would be the optimum.

As an added precaution I was hoping to be able to open the alternator excitation and dexcite the alternator in the event of a fault.
It wouldn't be quick enough to meet any disconnection times but would be useful in the case of an IT system as this would be right at the source in the generator terminal box.

Im torn which way to go with this.
I appreciate my application is not the most common

Thanks
 28 February 2013 10:08 AM
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AJJewsbury

Posts: 10964
Joined: 13 August 2003

Not really my area of expertise, but my thinking would be along the lines of having one approach that covered both when running from the onboard generator and when using mains - just from the economic point of view of not needing two sets of protective devices etc. and minimising complication. Since the mains supply will almost certainly be TN or TT, it might be simpler to have the on-board system TN as well (albeit "earthed" to chassis rather than true earth).

Others here have more experience of generators and probably mobile systems as well, so see what other posters say.

- Andy.
 01 March 2013 07:37 PM
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kevincullen900

Posts: 7
Joined: 25 July 2008

Thanks again Andy

I was speaking with the alternator manufacturer.
Most of their applications utilize an IT supply, as seems to be considered as safer over a greater range of applications.
As I mentioned earlier as the unit can be supplied from both the on-board generator and an external source this would leave the earthing system as IT on the generator side and TN (most likely) on the external supply side.
Perhaps as you suggest it would be simpler to have one system for both cases. I just cant make up my mind which way to go on this

Thanks
 01 March 2013 07:58 PM
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dlane

Posts: 685
Joined: 28 September 2007

Originally posted by: kevincullen900


As an added precaution I was hoping to be able to open the alternator excitation and dexcite the alternator in the event of a fault.

Thanks


From my experience I would not open the excitation to a generator as loss of excitation will cause a synchronous generator to then act as an induction generator and overheat.

Excitation supression on generators is carried out by shorting out the field to allow it to collapse and reduce the output of the generator. For larger generators the field is shorted through a disharge resistor.

Kind regards

Donald Lane
 03 March 2013 08:46 PM
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jcm256

Posts: 1793
Joined: 01 April 2006

Almost a similar problem to the one you describe at quarries (excluding external supply) that are moving away from fixed processing machinery to mobile machinery. Some mobiles are engine driven with hydraulic motors, but most now have onboard three-phase generator, driving jaw crushers, conveyor belts, screens etc.
Mobile crushing and screening plant has now become a preferred solution in many quarry applications.
With the ability to be set up quickly and the flexibility to manoeuvre around site, mobile plants can be used economically on a wide range of products in many quarries and demolition sites. Designed and developed for these reasons it is common to find mobile systems with up to three crushing stages for the production of high quality concrete and asphalt products.

Lack of guidance from HSE on earthing requirements for this particular type of machinery (so I cannot give you any advice) until it is forthcoming. Mostly depends on manufactures as fitted earthing that is what they supply. It is mostly an isolated earthing system for the machinery only, If a supply is required from the mobile plant (and there is sockets for this use) two phases is taken (no earth) to an isolating transformer, the secondary side mid point is then the earth for single phase or whatever required.

You say your on board machinery may be supplied on occasions from an internal supply, therefore your four pole change-over switch should be satisfactory , but if you leave the external supply connected during generator running your external earthing system would be connected . In the case of TT and IT systems, the MET should be earthed by an independent earth electrode only.


There is a solution for IT connected plant, yours is a bit small for the cost, look up, A-Isometer.


http://www.ussu.co.uk/ClubsSoc...F%20BS7430%202011.pdf


http://www.bender-ca.com/filea...Isometer_PROSP_en.pdf

http://www.bender-de.com/filea...wer-generators_PROSP_

Good luck to you, others are in the same boat to ok this stuff, on the fog index this is medium, but do your best.

Sorry not much help
Regards
jcm
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