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Topic Title: Voltage Drop from Source to Final Circuit!!
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Created On: 14 November 2012 12:47 PM
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 14 November 2012 12:47 PM
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altezzarm

Posts: 6
Joined: 09 August 2012

Can anyone please clarify the permitted maximum voltage drop available from a network source i.e Traffo?

I am generally used to designing from a 4-5% limit to final circuit.

I am being told now, I can design from Source (Traffo) using an 11% limit.

Can someone elaborate please?
 14 November 2012 01:18 PM
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Avatar for OMS.
OMS

Posts: 19549
Joined: 23 March 2004

Originally posted by: altezzarm

Can anyone please clarify the permitted maximum voltage drop available from a network source i.e Traffo?

Depends on the transformer ownership - if it's private (ie you have control over the tap changers)
I am generally used to designing from a 4-5% limit to final circuit.

I think you need to clarify that - in the absense of any other information then 3% (lighting) and 5% (others) is a reasonable guide in BS 7671.

Total voltage drop would probably be targeted at about 8% for the full system from source to end of index circuit. If you are allowing for growth (say 20% spare allowance) then 10 - 12% system voltage drop can be tolerated but noting final circuit limits above.

I am being told now, I can design from Source (Traffo) using an 11% limit.

You could if you have ownership of the transformer by adopting a quasi ESQCR limit of 6% and then adding a 5% limit to give you 11%.

Of course, all of this is based on you not knowing the installed equipment parameters in detail - if you do know the lower voltage limit then BS 7671 allows you to design for that. In practice, given that lots of CE marked kit will go down to 220V or less and we still have 240V at terminals then the design would need to be pretty shoddy for you to have a real problem



Can someone elaborate please?

See above - and welcome BTW



Regards

OMS

-------------------------
Failure is always an option
 14 November 2012 01:20 PM
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AJJewsbury

Posts: 11349
Joined: 13 August 2003

DNOs are allowed a 6% drop (it's 230V +10% -6%), so if you're in the situation where there aren't any DNO losses to consider, it makes sense to be able to work within the same overall range.

That said, table 4Ab seems to suggest 8% (or 6% for lighting) rather than 11%.

- Andy.
 14 November 2012 01:22 PM
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AJJewsbury

Posts: 11349
Joined: 13 August 2003

Bah, I was too slow again!
- Andy.
 14 November 2012 01:41 PM
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Avatar for OMS.
OMS

Posts: 19549
Joined: 23 March 2004

There are no winners here, Andy, what's two minutes in this grand life of ours - all have won and all must have prizes -

Regards

OMS

-------------------------
Failure is always an option
 14 November 2012 04:32 PM
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altezzarm

Posts: 6
Joined: 09 August 2012

Thanks for the prompt replies lads.

Extremely Informative!!

I suppose my confusion stemmed from always workin from a 4% (Ireland ETCI Reg's) volt drop to final circuit.

So for instance I have a Traffo -> MDB ->SDB -> Lighting Circuit

230 V system (Irish Nominal Voltage) @ 4%VD = Minimum of 220.8V @Lighting Circuit!!

Is the other 4% allowed for on the DNO side if theres a maximum of 8% from source?
 14 November 2012 04:44 PM
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AJJewsbury

Posts: 11349
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Both OMS and I were quoting UK figures: i.e. 3% for lighting, 5% for other uses when taken from a "normal" LV public supply. (6% and 8% from a "private" supply) and -6% from the UK statutory limits for supplies.

I guess the DNO will have to consider v.d. on their HV side too (Ohm's Law applies to them just the same), so perhaps some of the "11%" should be reserved to cover that, so perhaps 8% isn't an unreasonable target.

- Andy.
 14 November 2012 05:15 PM
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Avatar for OMS.
OMS

Posts: 19549
Joined: 23 March 2004

Originally posted by: altezzarm

Thanks for the prompt replies lads.

Extremely Informative!!

I suppose my confusion stemmed from always workin from a 4% (Ireland ETCI Reg's) volt drop to final circuit.

to clarify - are you saying 4% from origin to the protective device of the final circuit ? - I only ask as in the dim past of BS 7671 the 4% value was for final circuits, not the distribution up to the final circuit.


So for instance I have a Traffo -> MDB ->SDB -> Lighting Circuit

Being a a bit of a nutter when it comes to describing things ,

TX > Main Distribution Board > Final Distribution Board > Final Circuit

Source > say 1% > say 2% > say 3% at end
Voltage



230 V system (Irish Nominal Voltage) @ 4%VD = Minimum of 220.8V @Lighting Circuit!!

At the end of the lighting circuit I suspect you mean ?

Is the other 4% allowed for on the DNO side if theres a maximum of 8% from source?


The DNO side will vary of course - at HV as well (we all have to obey ohms law) - essentially those are the ESQCR values Andy and I suggested. Tjhe point being is that you own the transformer, you have control and can "tap up" the voltage so you could have say an equivalent of 8% from source to end of final circuit butstill have reasonably high voltage at that point anyway. You do really need to be careful though as the installation unlaods you can find your site voltage creeping up really high (with all the consequent problems).

I once managed to drop out a pretty big and important site when black start testing by operating the overvoltage shunt protection trips - even though I specified them, I forgot that we alos had about 1MW of standing load in normal operation - when I took it away, the voltage shor over 253V and everything went black.

Of course I claimed it was all part of the robust system testing and my protection design was proved - basically proving you can baffle most people if you keep a straight face and sound plausible -

I did get them reset at about 256V and scaled back on the tap change settings though, just in case that standing load did ever disappear one night -





Regards

OMS

-------------------------
Failure is always an option
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