Max Zs for Bs88-2 630amp fuse

[Martin Flint]

hi can anyone help I have a 3 phase bussbar protected by BS 88-2 fuses rated at 630 amps I need to know what is the max Zs allowed for this ?

 

[Wilko]

Hi - looks like 0.056 from Lawson spec, check link to confirm it's suitable
Lawson Fuses Application Data - http://help.tradingdepot.co.uk/electrical/lawson-fuses/lawson-fuses-application-data/

 

[Martin Flint]

Many thanks for your help

 

[Andy-1960]

Cooper Bussmann list it as 0.054 ohms for a 630A BS88 HRC fuse

 

[Wilko]

Cooper Bussmann list it as 0.054 ohms for a 630A BS88 HRC fuse

Reading it, the Lawson ref lists 240V and this may be 230V, if that makes any sense?

 

[Andy-1960]

One little word of warning, if you are measuring your Zs very close to a transformer, you may find you get false (high) readings due to the impedance of the transformer windings. I have done a little spreadsheet that will calc the Zs at the incoming terminals of a switchboard fed by a transformer. Let me know if you would like this.

 

[Martin Flint]

Hi I am getting a reading of 0-09 at top of bussbar 15 floors and max Zs is now 0.05 any ideas don't really want to install rcd as feeding floors in hotel also a CHP on roof with boilers ?

 

[Wilko]

Is there any on site doc from commissioning ? i.e. did it ever meet the 0.05 ?

 

[Martin Flint]

No sorry no documents so is the only way to make it comply is to use an Rcd ? If so can you advise me please

 

[westward10]

I agree with Andy you are probably not getting a realistic Zs. You may have to introduce a known resistance and deduct this from your reading.

 

[Martin Flint]

And you think the reading is not realistic because of the transformer ? How would you recommend putting a resistance on the circuit bearing in mind it's an open occupied hotel ! Sorry I've never come across this problem and not really tested bussbar

 

[westward10]

What Ze have you got.

 

[Martin Flint]

0.08 Ze

 

[westward10]

And you think the reading is not realistic because of the transformer ? How would you recommend putting a resistance on the circuit bearing in mind it's an open occupied hotel ! Sorry I've never come across this problem and not really tested bussbar

Accurately measure some 1.5 singles to say 0.10 ohm. Take a Zs at the busbar with meter leads only then take another reading with your known resistance in series with one of the meter leads and then deduct the 0.10 ohm and see if it differs from the original reading. This often works if you are very close to the transformer.

 

[Martin Flint]

Oh I see many thanks for your help