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Topic Title: MCCB's and Max Zs
Topic Summary: Ze higher than max Zs
Created On: 18 February 2012 01:06 PM
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 18 February 2012 01:06 PM
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primo

Posts: 426
Joined: 10 January 2008

How can you meet some of the very low max Zs figures for MCCB's unless you have a very low Ze?

With a TN-S supply using it's max allowed Ze you've got no chance, and even with a TN-C-S with a low Ze it can be a pretty close.
 18 February 2012 01:27 PM
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jleltd

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Buy MCCB's with adjustable magnetic trips and set the magnetic to suit the available Zs, with a little margin on the safe side for error. OK as long as no stupid inrush loads are used.

typo edit

-------------------------
James
 18 February 2012 03:18 PM
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daveparry1

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When you measure Zs all the bonding will be in place so it could well end up lower than the original Ze,

Dave.
 18 February 2012 03:51 PM
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OMS

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Originally posted by: primo

How can you meet some of the very low max Zs figures for MCCB's unless you have a very low Ze?

With a TN-S supply using it's max allowed Ze you've got no chance, and even with a TN-C-S with a low Ze it can be a pretty close.


Like any protective device, you need high current flow to operate it in a defined time.

For bigger devices, with correspondingly lower max Zs values, you are also likley to have larger capacity supplies, with correspondingly lower Ze values.

The DNO are not likley to offer you Ze of 0.8 or 0.35 ohms on a 630A supply for example.

If it's TN-C-S you could estimate it from the allowable volts drop - so for 630A and an ESQCR limit of -10% (ie 23V) then R=V/I = 23/630 = 0.037

If you assume (don't do this in practice) that an MCCB needs 10 x In for instantaneous disconnection then you need (for say a 400A device 10 x 400 = 4000A flowing.

The fault current would be 230/0.037 = 6216A - so no problem there even though you have a "large" MCCB

It gets a bit problematic on rural supplies but in general, if the DNO can deliver the supply capacity, they can also offer a low (and sometimes very low) Ze

Does that help ?

Regards

OMS

-------------------------
Failure is always an option
 18 February 2012 07:04 PM
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John Peckham

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OMS

I think that should be -6% not -10%?

The trouble with winding the magnetic setting right down to accommodate a high Zs is the MCCB is likely to pop on high inrush currents.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 18 February 2012 07:24 PM
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Fm

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Whats the handheld tester used on abb mccbs for again, saw one in a plant room lying about?
Fm
 18 February 2012 09:26 PM
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alancapon

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Originally posted by: John Peckham
. . . I think that should be -6% not -10%? . . .

Correct. Although the European standard was changed to -10%, the ESQCR regulations which govern the UK parameters etc remains at -6%.


Regards,

Alan.
 18 February 2012 10:01 PM
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primo

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Originally posted by: OMS

Originally posted by: primo



How can you meet some of the very low max Zs figures for MCCB's unless you have a very low Ze?



With a TN-S supply using it's max allowed Ze you've got no chance, and even with a TN-C-S with a low Ze it can be a pretty close.




Like any protective device, you need high current flow to operate it in a defined time.



For bigger devices, with correspondingly lower max Zs values, you are also likley to have larger capacity supplies, with correspondingly lower Ze values.



The DNO are not likley to offer you Ze of 0.8 or 0.35 ohms on a 630A supply for example.



If it's TN-C-S you could estimate it from the allowable volts drop - so for 630A and an ESQCR limit of -10% (ie 23V) then R=V/I = 23/630 = 0.037



If you assume (don't do this in practice) that an MCCB needs 10 x In for instantaneous disconnection then you need (for say a 400A device 10 x 400 = 4000A flowing.



The fault current would be 230/0.037 = 6216A - so no problem there even though you have a "large" MCCB



It gets a bit problematic on rural supplies but in general, if the DNO can deliver the supply capacity, they can also offer a low (and sometimes very low) Ze



Does that help ?



Regards



OMS


Yes, that does help, thanks. It's also made me re-visit the reason for having such Ze figures quoted in the first place!

I don't work on many large supplies and when I have I've not had to specify any distribution circuits on them. This came to light after using MCCB's to distribute a 69kVA TP supply around a site using MCCB's due to cable sizes involved and no. of sub mains - it was the most cost effective and practical way of doing it, not necessarily the best design solution! Anyway, in this instance the Zs for the MCCB's was fine but could have been an issue if the Ze was much higher. (Thanks to Alan for bringing this to my attention in a previous post!)
 18 February 2012 10:45 PM
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Inrush

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Correct me if I'm wrong but I thought most DNO LV supplies were obtained from TXs with a 433V secondary.

If this is the case then the DNO starting phase voltage would be 250V, therefore their maximum voltdrop would be 250 - 216.2 = 33.8 (assuming 230-6%), for a 100A supply:

33.8 / 100 = 0.34 Ohms
 19 February 2012 11:01 AM
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OMS

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Originally posted by: John Peckham

OMS

I think that should be -6% not -10%?

The trouble with winding the magnetic setting right down to accommodate a high Zs is the MCCB is likely to pop on high inrush currents.


LoL - OK John - ESQCR is -6%, but also +10%.

Given that we have to also have some scope within the installation, using -10% gives a more reasonable figure based on the supply voltage being higher than nominal voltage whilst leaving a bit of margin

I guess what I was showing was a simple device to answer the OP's question about low Ze and to demonstrate that bigger supplies would have lower Ze values - but you knew that.

Depending on the MCCB and the tripping module you can combat inrush problems on high(ish) Ze supplies by means of the adjustable short time pick up - effectively you slug the MCCB to ride through the short duration inrush, whilst letting it do the business for earth faults depending on the required disconnection time.

Same scenario for short circuits - your not limited by time so as long as the cable withstand is OK, you can have a very small multiplier of In to initiate tripping but you delay tripping (in some cases for many seconds) to agin give you selectivity.

Ultimately, you may well have tripping modules communicating with each other to ensure full selectivity across an installation - so called cascade protection settings.

Regards

OMS

-------------------------
Failure is always an option
 19 February 2012 11:05 AM
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Jaymack

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Originally posted by: Inrush
If this is the case then the DNO starting phase voltage would be 250V, therefore their maximum voltdrop would be 250 - 216.2 = 33.8 (assuming 230-6%), for a 100A supply: 33.8 / 100 = 0.34 Ohms

Interesting calculation. How can you account for Ze's of below 0.35? (0.20 say).

(The last time I looked), standard distribution transformers are designed with 5 taps giving ± 5%, with 2 steps of 2.5% up and down from the nominal 0%. The taps could be on any position, other than that giving maximum voltage, dependent upon the anticipated loading.

Regards
 19 February 2012 11:12 AM
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OMS

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Originally posted by: Jaymack

Originally posted by: Inrush

If this is the case then the DNO starting phase voltage would be 250V, therefore their maximum voltdrop would be 250 - 216.2 = 33.8 (assuming 230-6%), for a 100A supply: 33.8 / 100 = 0.34 Ohms


Interesting calculation. How can you account for Ze's of below 0.35? (0.20 say).

Bigger ampacity supplies Jaymack ?


(The last time I looked), standard distribution transformers are designed with 5 taps giving ± 5%, with 2 steps of 2.5% up and down from the nominal 0%. The taps could be on any position, other than that giving maximum voltage, dependent upon the anticipated loading.

Indeed, it was just a working example in the absense of any other information to show the OP that increasing supply capacity whilst maintained within realistic voltage tolerances will show a diminishing Ze - as you would expect due to bigger transformers and less distribution drops on bigger cables.


Regards


regards

OMS

-------------------------
Failure is always an option
 19 February 2012 11:36 AM
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alancapon

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Originally posted by: Inrush
Correct me if I'm wrong but I thought most DNO LV supplies were obtained from TXs with a 433V secondary. . .

Yes they do, because the "ESI Standard Transformer" is still specified with a 433v secondary.

. . . If this is the case then the DNO starting phase voltage would be 250V, therefore their maximum voltdrop would be 250 - 216.2 = 33.8 (assuming 230-6%), for a 100A supply:

33.8 / 100 = 0.34 Ohms

Not quite, unless it is a single property. As the loads are distributed along the length of the LV distribution mains, the contribution from each one has to be taken into account in the calculation. You also need to remember that with a balanced load, the current in the neutral will be low, hence there may only be volt drop to calculate in the phases.

In terms of the current used for the calculation, it almost certainly will not be 100A. For commercial premises, it will be the agreed ASC (Authorised Supply Capacity). For domestic, it will be a standard amount including "DNO diversity". This will vary between DNOs, and will obviously depend on whether the heating is electric or not. For a non-electrically heated house, values of 15A to 20A per connected property are not unusual. The diversity figure is based on an averaged load over 24 hours, which will allow for cable cooling time during periods of minimum load.

Certainly for domestic, the resistance of the supply is more likely a requirement to operate the cutout fuse in 5 seconds of a bolted fault appearing within 3m of the meter "load" terminals on the customer's tails, rather than any voltage drop issues.

Regards,

Alan.

Edit: last paragraph

Edited: 19 February 2012 at 11:48 AM by alancapon
 19 February 2012 12:54 PM
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primo

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Okay, so using the max allowable volt drop figures to work out Ze I understand. But if you have a supply that is 250v at the customers intake, does this suggest that the voltage at the transformer will be higher?

Or are you only likely to see 250v very close to the transformer? If this is the case then surely a supply with max allowable voltage will have and extremely low Ze? I have some real figures here... supply voltage at customers terminals is 251v with measured Ze of 0.14 ohms (100A TP), although at one point I did measure it at 0.07 ohms but this appears to only have been for one day and the DNO reading when installed was also 0.14.

Is it actually possible to calculate the Ze of this supply or can you only calculate using the -6% (or -10% figures!) for a worst case scenario?

This has also got me wondering how many 100A TN-S supplies I've seen (or not) as higher TN-S Ze values would not be suitable for earth fault protection with a 100A 1361 would it?!!

Confusing myself now...
 19 February 2012 01:24 PM
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OMS

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Originally posted by: primo

Okay, so using the max allowable volt drop figures to work out Ze I understand. But if you have a supply that is 250v at the customers intake, does this suggest that the voltage at the transformer will be higher?

It may well be, but that's a DNO affair based on tap selection and thier understanding of base load locally


Or are you only likely to see 250v very close to the transformer?

Well distribution cables obey ohms law and so have a volt drop under load - as you get further away, voltage decreases and Ze increases.

If this is the case then surely a supply with max allowable voltage will have and extremely low Ze?

It may have a lower Ze - but it may also be lightly loaded

I have some real figures here... supply voltage at customers terminals is 251v with measured Ze of 0.14 ohms (100A TP), although at one point I did measure it at 0.07 ohms but this appears to only have been for one day and the DNO reading when installed was also 0.14.

OK - your into a problem area - the volts drop will be across phases, but you are looking for Phase to earth - so assuming TN-C-s it's not easy to predict what loadinng the neutral has due to phase imbalance - which will influence the volts drop

Is it actually possible to calculate the Ze of this supply or can you only calculate using the -6% (or -10% figures!) for a worst case scenario?

Well you can estimate it - you could calculate it a little more accurately if you made voltage measurements on load and off load and you knew the load - which is basically what your loop tester is doing.


This has also got me wondering how many 100A TN-S supplies I've seen (or not) as higher TN-S Ze values would not be suitable for earth fault protection with a 100A 1361 would it?!!

Well not really, but again you are talking about the differnce between BS 7671 and ESQCR - the latter not having to deal with any defined disconnection times under short circuit or earth fault


Confusing myself now...

Not really - all we've done is use a bit of Ohms law to estimate a Ze and to show how assumed values of say 0.35 ohms are deduced and how they may well be much smaller for larger supplies and hence larger CPD's needing lower max zs figures being able to operate


regards

OMS

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Failure is always an option
 19 February 2012 03:27 PM
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jcm256

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Interesting your low down on Ze, good job it does not apply to quarries where you could have electrical apparatus working ¼ mile from the original low voltage supply.Zs could be as high as 2ohms from the original Ze of .01 ohms.
Not sure how this would pan out either, but BS7671 maybe next time
The following are some of the major changes: (Amendment No.1 to Fourth Edition) (Ireland)
. Chapter 41 and Chapter 61: Fault loop impedance tripping times are all 0.4 sec, the 5 sec time applies only to special cases e.g. distribution networks.

Wonder what way dear old Scotland would go if it ever happens in the next few years.
(IEC and CENELEC) or BS7671.
 19 February 2012 03:35 PM
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OMS

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Interesting your low down on Ze, good job it does not apply to quarries where you could have electrical apparatus working ¼ mile from the original low voltage supply.Zs could be as high as 2ohms from the original Ze of .01 ohms


That's just a function of design though - a long circuit is a long circuit and needs to be designed based on the CPD requirements

TT and local electrodes would solve most of the problems

Regards

OMS

-------------------------
Failure is always an option
 19 February 2012 05:05 PM
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Jaymack

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Originally posted by: jcm256
Wonder what way dear old Scotland would go if it ever happens in the next few years. (IEC and CENELEC) or BS7671.

We would hang on to the things that work in Grosse Britannia, we'll let Part P remain where it is, although we do require more control of the merry band of rapscallions cum electricians in Scotland. Colours may change though, I see tartan and tweed as new ones!

Regards
 19 February 2012 05:08 PM
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alancapon

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Originally posted by: primo
Okay, so using the max allowable volt drop figures to work out Ze I understand. But if you have a supply that is 250v at the customers intake, does this suggest that the voltage at the transformer will be higher? . . .

As OMS says, the distribution feeders will obey ohms law, so the voltage will be higher at the transformer under most circumstances (additional sources of supply such as small scale wind and solar generation can change this). If you are fed off the distribution network, then the DNO can rearrange the network for either fault repair or routing maintenance without warning. The arrangements can quite justifiably leave 216.2v at the customer's incoming terminals.

Regards,

Alan.
 22 February 2012 07:26 PM
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lyledunn

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Ground fault protection on the mccb is a simple method of dealing with the problem where very low values of Ze are required. We have used them in quarry work with no ill effects from nuisance tripping. Adjustable setting but normally at 3000mA with time-delay if required.


-------------------------
Regards,

Lyle Dunn
IET » Wiring and the regulations » MCCB's and Max Zs

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