LOW ZE READINGS

[Farmelectrics]

carrying out an eicr on a building recently we found the external ze reading to be extremely low ie 0.02ohms. so by measurement and testing are fault current read 20ka. now most of the mcbs rcbos ets have an icn of 10kohms. this poses a problem as obvisoluy the mcbs etc would self destruct under fault conditions. what would you code this and more importantly how would we rectify this problem, Interested to here your thoughts.

 

[Wilko]

Hi - Some testers can be inaccurate at very low values, so I’d be redoing the measurement with a known length of cable added in series and seeing if it stacked up. I have a Long Lead that’s about 1 Ohm that I’d use for this.

 

[James]

2 options available,

1 extend the supply cable by running a length of swa from incomer to fuseboard, 50m would probably bring the reading down enough.

2 change the breakers to a higher rating of fault current.

 

[Farmelectrics]

Wilco it is pretty close to the sub station however I've always been a great believer the meter never lies even checked the meter on my cal card all readings very accurate.

 

[Strima]

3. Test using three lead no trip and get a higher Ze reading therefore lower PFC... ??

 

[James]

I had a case where I was measuring about 3m away from transformer and the meter was giving readings that were far higher than I expected.
changed the values on cert. to what the transformer manufacturer stated.

 

[Farmelectrics]

are you suggesting I cheat and higher the ze reading. ha ha

 

[Farmelectrics]

all the mcbs are 10k icn would be extremely expensive to change them what code would you give to this

 

[Julie.]

The reading may be perfectly correct, and the mcbs ok as well.

If the installation is protected by fuses, the let through may be less than the rating of the downstream mcbs.

Just check the fuse let through first.

 

[James]

are you suggesting I cheat and higher the ze reading. ha ha

not cheat,
figure can be found by measurement or investigation.

if the transformer manufacturer says max pfc is 20Ka and the meter reads 92Ka then the manufacturer wins on my paperwork.

all the meters suggest that they may be inaccurate when close to transformer but not 1 of them says in the instructions that below x meters away you should ignore the reading on meter!!