Maximum Zs exceeds manufacturer recomendations

[KiltedSparky]

Hi,

we have a 400A MCCB Bs60947-2 installed and the Zs reading measured at device is 0.2 ohms the manufacturer has listed the Zs value for this MCCB at 0.045 ohms , i have been informed by designer of circuit that because the Earth fault current is known to be 1537A he has just to tweak a setting on breaker which is setting the Instantaneous element to 3.5 X 400 =1400A and this will suffice does this mean that the we do not require to adhere to manufacturers tabulated values or is designer wrong

hopefully someone can advise :smiley2:

 

[IQ Electrical]

Hi,

we have a 400A MCCB Bs60947-2 installed and the Zs reading measured at device is 0.2 ohms the manufacturer has listed the Zs value for this MCCB at 0.045 ohms , i have been informed by designer of circuit that because the Earth fault current is known to be 1537A he has just to tweak a setting on breaker which is setting the Instantaneous element to 3.5 X 400 =1400A and this will suffice does this mean that the we do not require to adhere to manufacturers tabulated values or is designer wrong

hopefully someone can advise :smiley2:

If the device has an adjustable multiplier then the designer might be correct but it's no good measuring the Zs 'at the device' it needs to be measured at the extremity of whatever circuit it is supplying.

 

[KiltedSparky]

Apoligies IQ that was a typo , the Zs was measured at the sub mains panel which the MCCB was supplying .The only problem i can see with adjusting the multiplier to suit the measured Zs reading is that when a periodic is carried out in 5 years time, surely the company carrying this out will rely on the manufacturers published readings and fail this circuit as a CAT 1 fault.

 

[IQ Electrical]

The competence (or lack of) of any future inspector is not your concern, if the design is correct and the installation corresponds with that design then you have done your duty.

 

[ACDC]

Just write a comment on any certification/documentation explaining the reasons for the settings.....Would also place a tamper proof sticker over the dials of the MCCB, should be covered then.

 

[KiltedSparky]

Just write a comment on any certification/documentation explaining the reasons for the settings.....Would also place a tamper proof sticker over the dials of the MCCB, should be covered then.

Thanks AC , very useful advice . We are going to obtain a letter from designer so that it can be inserted into file with test certs but we never thought about the tamper proof sticker, that as you say should cover us when periodic is due