Ze / PFC conundrum at factory

[Dunc88]

Hi all,
I recently went to install a new circuit & when trying to obtain a ze reading at the origin (TN-C-S) I got 0.01 ohms with a pfc of >19.9kA... This is when testing from the main distribution board which is in a hut next to the building where the work took place 15/20 meters away. Anyone came across this type of scenario before!?
Cheers

 

[Wilko]

Hi - perhaps a measurement issue. You could remeasure with a known R in series and see if it adds as it should?

 

[westward10]

Modern testers are not good at measuring low impedance values of Ze caused by being close to the substation of which a factory may have its own. As Wilko said introduce a known resistance to the test and deduct this from your reading but if you are close to the source substation it will be low.

 

[Dunc88]

Cheers Wilko, Westward... when I test from DB1 in the factory itself, the readings go back to normal which tells me its not my tester. It is close to the substation actually, didnt realise this would give me a high pfc & low ze! So a possible option to find a more accurate reading would be to calculate the difference between the R1+R2 & Zs @ the first DB in the factory? If so, how would I go about calculating a lower pfc taking into consideration its a three phase supply?

 

[The Ghost]

Yes I have come up with low Ze in a domestic. Sub station 100 yards away. If you look at your manual you may see it specifies that the reading you are taking is within six figures. Meaning your reading may be .07. I or the customer had a new cut out by the DNO and when they read it it was .11 (Ze) I spoke with the tech at Kewtech and they pointed the figures thing. So my reading was .03 and of course there were concerns regards the Ics/Icw of the MCBs. But if I/you think about it if I add the six figures I am not that far away from the DNO figure. It would seem we are at such a fine level of measurement it is very hard to really get an accurate figure in those circumstances.

 

[Ian1981]

I wouldn't worry about the ka ratings of the mcbs too much as back up protection can be with manufacturers guidance be achieved by the fuse in the cut out which will clear the fault

 

[westward10]

Cheers Wilko, Westward... when I test from DB1 in the factory itself, the readings go back to normal which tells me its not my tester. It is close to the substation actually, didnt realise this would give me a high pfc & low ze! So a possible option to find a more accurate reading would be to calculate the difference between the R1+R2 & Zs @ the first DB in the factory? If so, how would I go about calculating a lower pfc taking into consideration its a three phase supply?

You could use the R1+R2 of the board submain but I would personally use a known resistance of say 0.10 and introduce this to the test then deduct it. It is bound to be low if you are in close proximity to the transformer.

 

[Dunc88]

Ok great, so would you record the ka as you get it at the main consumer unit?

 

[westward10]

Calculate it from your Ze. My old Robin tester rarely gets thrown by low impedance Ze, don't make em like they used to.

 

[Dunc88]

Calculate it from your Ze. My old Robin tester rarely gets thrown by low impedance Ze, don't make em like they used to.

Ok great, when you say calculate with a ze of 0.01, how do you go about this to get a relative kA reading?

 

[westward10]

Use ohms law, divide it into the measured nominal voltage.

 

[Risteard]

I wouldn't worry about the ka ratings of the mcbs too much as back up protection can be with manufacturers guidance be achieved by the fuse in the cut out which will clear the fault

The conditional rating is 16kA so beyond that you cannot assume that they will withstand the fault current. I would ascertain the figures by enquiry - measurement simply won't be accurate.

 

[Ian1981]

The conditional rating is 16kA so beyond that you cannot assume that they will withstand the fault current. I would ascertain the figures by enquiry - measurement simply won't be accurate.

A type 2 bs 1361 for example has a typical ka rating of 33ka so I think it will.
Like I've said manufacturers should be consulted if back up protection is used

 

[Dunc88]

Use ohms law, divide it into the measured nominal voltage.

Yeah I did but got over 40kA still and was wondering if i'd missed a trick is all!

 

[Ian1981]

Yeah I did but got over 40kA still and was wondering if i'd missed a trick is all!

How did you get 40ka when if using nominal 230 volts
230volt/ 0.01=23000amps 23ka?

 

[Dunc88]

How did you get 40ka when if using nominal 230 volts
230volt/ 0.01=23000amps 23ka?

I divided 400v instead of 230v! well spotted!

 

[Ian1981]

I divided 400v instead of 230v! well spotted!

If it's 3 phase then it could be more like 40ka as you would do a pssc between live and neutral and double it or if your meter allows test between phases.

 

[Dunc88]

True, thanks for the help all. I'll be going back next week to finish off. One more thing (i'll try not to make it sound too confusing), i'm carrying out an installation cert (see the consumer unit layout below)
Consumer unit @ origin (4way tp) > (feeding) DB 1, & DB 2 (via 2x D63a mcbs)
-DB1 feeds new circuit 1 out of 2.
DB 2 > DB 3 & 4 (via C63a mcb) *this 4x16mm swa "piggy backs" in & out of DB3's main switch before entering DB4 (perfectly acceptable I presume but not something i've seen before?)
-DB3 feeds new circuit 2 of 2.
My main question is, with the continuation sheets I currently total 12 pages, this is because the only consumer unit I havent worked on is DB4, so I was hoping not to have to record any readings at this consumer unit & to make a note in comments instead saying "DB4 not tested" but in all honesty, because the supply from DB2 ends @ DB4 (piggy backed into main switch @ DB3) this is where I should record another ZS on a seperate continuation just to segregate the 2 boards, would you agree? If DB3 fed DB4 via a circuit breaker, could I then write on the circuit details of DB3;
Circuit details for new circuit installed & then
"all other circuits not tested"?
Sorry to ramble on!

 

[Wilko]

If I've understood correctly... it's a new supply to DB4 but no changes in DB4. If so, perhaps an update to the DB4 details showing the new Zdb ?

 

[westward10]

No way I would "piggy back" one board from another.