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Topic Title: Distance between transformer low volatge side and LV switchboard
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Created On: 11 June 2013 10:23 AM
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 11 June 2013 10:23 AM
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power98

Posts: 10
Joined: 22 March 2011

Hello Everybody,

I'm asking if it's mandatory to install ACB between the transformer low voltage side and low voltage switchboard main incomer (for feeder protection) if the feeder length exceed a certain limit.

Please, provide the reference in the BS or IEE.

Best regards,
 11 June 2013 10:08 PM
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MickeyB

Posts: 181
Joined: 18 January 2003

Not mandatory per se.... it's common practice for the LV tails to be 'unprotected' from an LV protection perspective save maybe a single pole Restrictive Earth Fault (REF) relay to protect the LV windings and the LV tails, but I have seen just HT protection.

The HT/ MV protection would offer some basic fault protection of the LV tails, depending on the protection type TLF, IDMT relay etc...
If HT only just make sure you have enough fault current on the HT side and fault withstand on the LV tails to take the fault for the length of time it takes the HT protection to trip..... especially if you're using TLF.
 12 June 2013 12:16 PM
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Avatar for timothyboler                                      .
timothyboler

Posts: 227
Joined: 25 July 2008

No regulation that I know of except that it's genrally bad design practice to have LV distribution transformers significant distances away from the load centres/main switchgear owing to the current and cable sizes involved. How far are we talking? I've seen a >100m run at 4000A once - what a waste of money!

Regards, Tim

-------------------------
Everyone loves a fireman - but hates the fire inspector.
 12 June 2013 08:57 PM
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OMS

Posts: 18917
Joined: 23 March 2004

No statutory distance I'm aware of

Length could be dictated by physical restrictions, fire engineering or the type of building supplied - sometimes seperation is a good thing.

Standing voltages tend to become a problem over longer distances

You just need to ensure that you have protection on the LV side - how you do that is just a matter of design and could make use of the HV protection

Regards

OMS

-------------------------
Failure is always an option
 12 June 2013 09:11 PM
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cookers

Posts: 203
Joined: 10 February 2012

I agree with the others, there is no mandatory rule I know of.

I usually have a think about this if the tails get longer than 10m.

Usually if route is simple and all in "secure" plant areas then don't fit, if cable goes all over the place then fit CB on Transformer.
 13 June 2013 07:13 AM
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power98

Posts: 10
Joined: 22 March 2011

Thanks.

Is there a reference recommend this 10 m?


Best regards,
 13 June 2013 02:36 PM
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Avatar for timothyboler                                      .
timothyboler

Posts: 227
Joined: 25 July 2008

What type of protection are you trying to achieve? For overload Reg 433.2.2 allows the overload protection to be located downstream (no max distance) as long as there are no branches etc. and is protected against fault current. This is because the cable won't overload unless the load connected to the switchboard overloads too. Fault current protection can - with care - be provided by the HV as OMS says although depending on the inrush current may be difficult to achieve thermal protection on the LV side using HV protection.

If you are worried about earth faults (probably the most common type of cable fault?) you can install restrictive earth fault protection (REF) in the switchboard and by putting the neutral current transformer in the transformer terminal box (with a pilot cable back to the switchboard) you can protect the whole length of cable i.e. looking backwards towards the transformer. There are different ways of doing this though.

Regards, Tim

-------------------------
Everyone loves a fireman - but hates the fire inspector.
 14 June 2013 03:18 AM
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jackdaniel

Posts: 69
Joined: 29 November 2012

Not mandatory, but from my experience, its not standard practice, My opinion, the protection of the primary side will protect the secondary side.We have the switchgear equipped with OC/EF..
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