Parallel paths headache.

[Big n Daft]

We can quote the bible all day. But when all's said and done we live in the real world, test results should reflect the "in service" real world, its is what it is.

 

[Thomas123]

Engineer 54.

To tell the truth the majority of my machines are only 3phase so a 4core cable would normally suffice(spare core being used as dedicated earth).Me, being a forward thinking chap though, I always install 5-core and terminate the neutral too in case we change machines in the future (or need a neutral for something else).
Like you say my fuses are normally 80kA but while I'm doing a Zs I always take another 20secs and do a PSCC and record the highest on my forms..........pedantic maybe but while I'm there it doesn't take long and it also helps me check against previous results as all the busbars are bolted together(and old) and any dodgy joint in the neutral loop would show up with a higher test from previous.
My machines have indeed Moulded circuit breakers and not Mcb's as I first stated.

It might just be me but my report sheets are turning into 'War and Peace' replicas

he is tony cable

 

[IQ Electrical]

Hi Eng 44,

PEFC and PSCC from all 'relevant points'.
1). Busbar tap off box fused at 100Amp BS88-2.
2).Isolator for D/Board fused at 80Amp BS88-2.
3).4 Way D/Board fused at 63Amp BS88-2.
4). Wall Isolater feeding machine transformer fused at 50Amp BS88-2.
5).Transformer Isolator 100Amp MCB
6).Machine Isolator 100Amp MCB 100mA RCD.

I treat each one as a seperate circuit as they all have circuit breakers with different breaking capacities and therefore I take readings at each point and record the highest value ( times 2) on my form.
The machine isolator for example has a capacity of 7.5kA.
Because my Zs is so low due to parallel paths my readings here are so good I was nearly having to upgrade the breaker.
I also find it saves on paperwork if I put all these test results on the same sheet and treat it as a full circuit instead of doing them all seperately.

Hope you understand my reasons and look forward to any suggestions you may have.

Allan

Lol, lovely discrimination in that lot!

 

[44ALLAN]

IQ Electrical

Re: Parallel paths headache.
Lol, lovely discrimination in that lot!
​

I would love to hear your thoughts on the way I have 'discriminated' IQ.

Machine supplier........well known cnc manufacturers said they needed fused machine Isolator @ 50 Amp'
I did that and everything else up stream of that............busbars being 'limitation without a cherry picker' I fused at 100Amp as that is the largest tap-off box available for the type of busbars and fused the next 'lower down' wall Isolator in -line @80 Amp then the D/b @ 63Amp.........'hoping/expecting the 80 or the 63 would blow before the 100 in the event of a fault......naive or what ( knowing full well its sods law in a fault situation what blows and what doesn't depending on where it is.........been there seen it etc)

They then supplied the 100A transformer with MCCB @100A and machine Isolator MCCB @100A...............(mebbes something to do with inrush currents of transformers would knock anything lower out...........as in trip... but you would obviously know that

I made the list of my install and I'm perfectly happy for you or anyone else to show me the errors of my way.
Never to old to learn so show me how you would have done the install.
I'm all ears.
TIA

 

[44ALLAN]

he is tony cable

Thanks for that comment even though I have no idea what it means...........care to explain?

 

[IQ Electrical]

IQ Electrical

Re: Parallel paths headache.
Lol, lovely discrimination in that lot!
​

I would love to hear your thoughts on the way I have 'discriminated' IQ.

Machine supplier........well known cnc manufacturers said they needed fused machine Isolator @ 50 Amp'
I did that and everything else up stream of that............busbars being 'limitation without a cherry picker' I fused at 100Amp as that is the largest tap-off box available for the type of busbars and fused the next 'lower down' wall Isolator in -line @80 Amp then the D/b @ 63Amp.........'hoping/expecting the 80 or the 63 would blow before the 100 in the event of a fault......naive or what ( knowing full well its sods law in a fault situation what blows and what doesn't depending on where it is.........been there seen it etc)

They then supplied the 100A transformer with MCCB @100A and machine Isolator MCCB @100A...............(mebbes something to do with inrush currents of transformers would knock anything lower out...........as in trip... but you would obviously know that

I made the list of my install and I'm perfectly happy for you or anyone else to show me the errors of my way.
Never to old to learn so show me how you would have done the install.
I'm all ears.
TIA

Hi Allan

I was certainly not criticising your installation, discrimination is not an exact science and it sounds like certain parameters were imposed on you anyway by means of manufacturers requirements and existing distribution equipment!

In most cases, full discrimination is expensive and not really necessary unless danger is introduced by the absence.

 

[44ALLAN]

I never thought for one second that you were mate...............well maybes the 'lol' bit about discrimination instead of telling me why you were laughing..........makes me look like a bit of a prat to tell the truth !
I know it wasn't intended by your response............but..........

How would you have done it?
I really want to know.

you said 'and it sounds like certain parameters were imposed on you anyway by means of manufacturers requirements and existing distribution equipment'!

How would you have done it............

Busbar fuse?
Next Isolator fuse feeding D/b?
D/b fuses?
Wall Isolator fuse?
Machine transformer MCCB is a given.............from manufacturer
Machine Isolator MCCB is a given.............from manufacturer

Do I query the manufacturer about their 100A MCCB's or take it they know what their talking about as they sell XXXXXions of machines all over the world?

I really do check their data though believe it or not to see that it has the correct kA and Zs rating..............sad eh !!!!

I'm really interested in knowing how you would have 'protected' these circuits as opposed to the way I did it.
Looking forward to your response and please don't be gentle with me.............if I'm wrong I want to know.

Edit

Lol, lovely discrimination in that lot!

Why the Lol ?
What have I done wrong?