Grading of HRC fuse with MCB 2

[SimonUK]

Hello

I am doing a desktop study looking at grading between HRC fuses and downstream MCBs. The upstream fuses tend to be GEC Alstom type 'T' HRC to BS88 and I have obtained data including the 'pre-arcing' values.

For the downstream MCBs, I need to make sure that at the particular DB fault level, the let through energy of MCB is lower than the pre-arcing of the fuse so that the MCB goes first as intended.

The problem I have is that the downstream MCBs are GEC Vynckier and I have not been able to obtain any information on these (even from GE direct).

Q1 - Can anyone point me towards any information on GEC Vynckier or similar Type 2 MCBs so that I can make this assessment?

I have done some calcs with Memshield data and this is showing a very high number of locations where there is no grading; eg with a 63A fuse upstream and a 32A MCB downstream there is only grading to 1.5 kA FL. On my site there are a lot of DBs with fault levels up around 7 – 10 kA (three phase – though most of the MCBs though are single phase).

Q2 - Does 63A immediately jump out as being far too small a fuse for a 32A downstream MCB?

Q3 - any other info on the issue of grading fuses with MCBs would be appreciated.

Thanks in advance



Simon

 

[7anoter4]

HI Simon
For type 2 MCB [BS 88 Part 2] see:

http://www.tlc-direct.co.uk/Book/3.6.4.htm

From table 3.5 for Imax=7*32=224 A will trip for sure in 0.1 sec [100 ms]
HRC 63 A Pre-arcing time for 224 A will be 20 sec
SEE

http://www.biagmbh.com/katalog/pdfs/96/9691.pdf

HRC Fuse according to BS 88 has a breaking capacity of 80 KA so it can break 7-10 KA as expected.
This is my opinion.
Btw Vynckier is a Belgian company.
Regards

 

[ScottyUK]

Type 2 MCBs are not listed under BS 88: BS 88 is a fuse standard. BS 3871 covers older MCBs with numerical designations Types 1 - 4, and BS 60898 is the modern equivalent which uses letters as designators Types A - D. Type 1 and Type A were / are uncommon and some manufacturers don't bother making them.

Grading problems aren't uncommon when using a mixture of fuses and MCBs. Normally use one or the other, or accept that under certain conditions you will lose discrimination. You can argue that fault level drops away pretty quickly as you move away from the board and that you will discriminate under most real-world faults. A maximum-level bolted fault in a cable is unusual - most have some impedance and the fault level is reduced to the point where the MCB will clear first. It's a pain in the ass if the upstream fuse was weakened though!

A 63A fuse with 32A MCB looks like it will not discriminate correctly at high fault levels from a quick glance at the curves. The different manufacturers all design their devices to meet the standard so time-current responses should be fairly similar even if you're using a different branded MCB in the simulation. Getting the right device model becomes more important with MCCBs which have adjustable tripping units of varying levels of complexity. If you want to show discrimination your options are pretty limited: either increase the upstream fuse size or consider an all-fuse installation.


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If we learn from our mistakes I'm getting a great education!

 

[SimonUK]

Thanks for the responses guys, very useful.

Scotty, when you say "from a quick glance at the curves" I take it you are referring to the curves in the meshield guide?

There may be an option to increase eg the 63A fuse to 100A - this would still protect the cable and put a bit of distance between the fuse and MCB size.

Its just a bit frustrating not having the detailed let-through energies for the Vynckier MCBs (for single phase and three phase faults) as it would allow a more precise assessment to be made ie direct comparison with the type T pre-arcing values.

Thanks again


Simon

 

[7anoter4]

You are right Scotty I gave for the MCB the BS for the fuse, by mistake.
But, as a rule of thumb, 2 levels up for the fuse with respect the MCB is fair.
From 7*32=224 A up to about 800 A the MCB will trip first in, let's say, 0.1 sec and from here up to 10KA or more the fuse will take the job.
I don't think that the MCB for 32 A will supply more equipment except an induction motor or a lighting panel so the maximum damage will be on the cable. The minimum cross section for a cable xlpe insulated, in air, will be about 14 awg. This cable is able to withstand 800 A for 0.1 sec even the short circuit start when the cable was warm[ 90o C].I think 63 A fuse upstream and 32 A MCB downstream will be fair.
Best Regards

 

[ScottyUK]

Hi Simon,

I hadn't looked specifically at the MEM website, only the generic curves. From a closer look at the MEM site I found this table that youy might not have seen:

http://www.memonline.com/cb3.html

from their data you need a 160A upstream fuse for guaranteed discrimination with a 32A Type B, which is in almost all respects the same as the earlier Type 2 MCB. Worse than I thought!

Hi 7anoter4,

The rule of thumb to maintain a 1.6:1 ratio as the absolute minimum between series devices assumes that the curves are similar in shape. The fuse curve and MCB curve are so different in shape that discrimination becomes a real problem at high fault levels. On large industrial and utility equipment the protection relays have an 'Extremely Inverse' curve which is good for discriminating with fuses, but not great for much else.


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If we learn from our mistakes I'm getting a great education!

 

[7anoter4]

Thank you, Scotty. It is surprising, but it is very interesting indeed!

 

[slavag]

Hi.
Some additional information.
Sorry for qulity.
Regards.
Slava

 

[slavag]

And next one.

 

[7anoter4]

Slava you are great!

 

[slavag]

Thanks a lot 7another4.
I take this information from ABB switchgear manual ( section 7).
I've hardcopy, but guys here also found this book in google.
about this book please see

http://www02.abb.com/global/seitp/s...9e7a3c1256f5d004a29bb/$file/index_abb_en.html

I think possible work online or order it.
I'm sure you can found many usefull information.
I think Marmite used section 12 from this book in your discution about reactors.
Best Regards.
Slava

 

[7anoter4]

Thank you Slava for the link. A friend of mine has a 9th edition Switchgear Manual of ABB. Yours seems to be 11 or 12 edition. For the time being, I think, this old edition is enough for me.
Regards