How Low (Ze) can you go?

[flyingspark]

Hello,

I took a reading of 0.07 ohms for Ze on a TN-S supply last week.

I have'nt had one this low before, double checked it and still the same.

I know max. allowable is 0.8 ohms on TN-S, so am wondering if anyone has any guidelines on how low can you go before suspecting something isnt quite right.

Many thanks

D.

 

[batty]

On my old di log meter 9083p I was getting 0.00ohm loop at mains I thought something was a bit wrong with it when I got a similar reading on a tt system. It has been replaced with a fluke now.

Batty

 

[seajayess]

It will all depend on were the transformer is to how low it will be, what was the PFC at the installation CJS

 

[green-hornet]

I have a fluke 1652 which I use for all my tests, however being old I still have my original seperate testers, if I get an anomoly like this I cross reference it with my old testers. However the old testers are not now calibrated so could not be used on an eic.

For a TN-S with a reading of 0.07 I would be very suspicious, borrow a tester from another electrician just to confirm the readings you get.

The lowest reading I have ever had for a TN-S is 0.11 and that is rare its normally 0.27 or above.

Remember to remove all bonding and test as close to origin as possible.

I have in the past removed the main fuse and tested at origin inside the cut out.

However I would not suggest you do that it is illegal to break any seals or interfere with the supply.

 

[Andy™]

depending on transformer location, cable size etc, you could well have lower than 0.07.

obviously, if you have a very low Ze, you will have a higher PFC and may need 10KA MCB's instead of the standard 6KA

For domestic, i rarely see lower than 0.25 for TN-S

 

[green-hornet]

Good point you raised there Andy, most mcb's are 6Ka. You have to do the PFC at the CU and take the larger reading, that is live to earth and live to neutral.

It would be rare outside London to get a situation where you need 10 KA breakers but this is where accurate tesating comes into force.

In my area I often see 0.17 and 0.27 for a TN-S and I have recorded 0.11 but this is very rare for TN-S.I have also recorded 0.36 or 0.37 on a pme.

It appeared the property was end of line on a new build, I reported my findings to the local authority and they said there was no problem.

So I had a EIC stating the Ze by enquiry as 0.35 and the actual reading of 0.37!

I just put in the statement that I contacted the authority and they are aware of the readings, just to cover my ass.

Most domestic PFC in my area are never more than 2KA, then again I have never had a reading of 0.007 for Ze

 

[Andy™]

Highest PFC i can remember was somewhere around 5KA. property with substation approx 5m away

even if the PFC is more than 6KA, you could still use 6KA breakers since they are backed up by a BS88 or 1361 or other

But, if the PFC is near to 6KA (maybe 5KA?) then the breaker will open but may not close, so might be an idea to use 10KA so breaker doesnt need replaced

 

[flyingspark]

Thanks for all the replys -

Batty- My Megger MFT1553 is only 2 months old, and checking it against my check box, its all good.

Seajayess - The PFC there was 3.2KA.

Am trying to book a time to go back with the client and look into it some more, will re- post with any findings.

Thanks once more.

 

[seajayess]

What meter are you using, if its a Fluke you should zero the leads out with them all connected together. I use a Fluke and before I zero the leads out I push the zero button with nothing connected to clear any readings that may be saved in its memory, was told that they can over calibrate and actually decrease the measurement CJS

 

[Robojin]

On my own supply TN-S I was given 0.17ohms by the DNO when they did some work and cut the cable in the garden, when I measured it inside I got 0.21ohms

I also have a 254volt at the supply, nearest sub is around 100m away

 

[Evans Electric]

Sounds low to me !! Its not something daft like the decimal point has moved along ?

Just a thought.

Deke

 

[Andy™]

place im working at this week has a Ze of 0.08. between phase and neutral is 0.07

 

[flyingspark]

Ok, so went back to the location where I got the .07 ohms reading for Ze, double checked everything, nulled leads etc., and still the same. So I called DNO (EDF) and they said its ok. Since then, today infact, I got .08 ohms, so I am not going to worry any more.

Thanks to all those who contributed to this,

Flyingspark.

 

[SPECIAL LOCATION]

Ok, so went back to the location where I got the .07 ohms reading for Ze, double checked everything, nulled leads etc., and still the same. So I called DNO (EDF) and they said its ok. Since then, today infact, I got .08 ohms, so I am not going to worry any more.Thanks to all those who contributed to this,

Flyingspark.

And thank you or the follow-up feedback...

Its nice to hear what people find out after question!Guiness Drink

 

[steptoe]

Highest PFC i can remember was somewhere around 5KA. property with substation approx 5m awayeven if the PFC is more than 6KA, you could still use 6KA breakers since they are backed up by a BS88 or 1361 or other

But, if the PFC is near to 6KA (maybe 5KA?) then the breaker will open but may not close, so might be an idea to use 10KA so breaker doesnt need replaced

NO you cant, you have no idea what they are protected by, could be a 6" nail.

you must put on the cert that the protective device you have used is capable of protecting the circuit.

no one else can be liable for something you have tersted and certify to be correct/safe and legal. this is one of the reasons you must be competent and able to stand in court and defend your test results.

in all honsety for you to say

well your honour, the DNO should have put a proper fuse in and stopped the little kiddie being fried is a joke.

 

[Andy™]

just what i was told at college... providing there is at least 1 device that can break the full PFC, then other devices can be rated lower breaking capacity. got to admit its not he best setup, and id rather use breakers that can take the PFC

But what if you have a factory next to the substation, that has a very high PFC, (12KA for example). are you expected to use a board with BS88's because a normal 10KA wont take it, even though its backed up with a BS88/1361 that will

(can you get normal breakers that have a rating more than 10KA?)

 

[steptoe]

point taken andy,

but the circuit must be protected by a device that you have installed, IMHO

not some third party device.

and yes, if you have a pfc of 16kA then you need HRC everywhere. 1361 will be fine tho, 16.5 kA and DNO state that PFC never exceeds 16, so you will be fine.

 

[Andy™]

point taken andy,but the circuit must be protected by a device that you have installed, IMHO

not some third party device.

and yes, if you have a pfc of 16kA then you need HRC everywhere. 1361 will be fine tho, 16.5 kA and DNO state that PFC never exceeds 16, so you will be fine.

Dont really see a problem with using a '3rd party' device for backup (providing you know that it is actually a fuse and not a solid link/nail/bolt. PFC is worst case, and unless something major happens, then its unlikely that fault current will be full PFC, so chances are a 10KA will break it safely

If your still paranoid about using a DNO fuse as backup, you could always install your own switchfuse with BS88's in feeding DB with 10KA's

 

[SPECIAL LOCATION]

just what i was told at college... providing there is at least 1 device that can break the full PFC, then other devices can be rated lower breaking capacity. got to admit its not he best setup, and id rather use breakers that can take the PFCBut what if you have a factory next to the substation, that has a very high PFC, (12KA for example). are you expected to use a board with BS88's because a normal 10KA wont take it, even though its backed up with a BS88/1361 that will

(can you get normal breakers that have a rating more than 10KA?)

Regs 133.2.2 is the general current capacity rule ref circuit breakers capable of carrying PFC..

Then in protection against fault current section 434

and reg 434.5.1 second paragraph refers to the point Andy is making.

"Lower PFC rated device acceptable if Higher PFC rated device on supply side."

as another point...

432.3 refers to devices for protection of fault currents being capable of breaking

AND for a circuit breaker making the fault current up to the PFC..

e.g. when switching an MCB if circuit has a direct short across it..

the MCB must not damage your hand whilst you are grabbed hold of the switch.

I did that once with a 40A MCB switched it on with a dead short across L-E

(absent minded spark had left a test lead across some cables!Blushing:O)

It don't half go pop sharpish when you try to switch it on! ] :^O:^O

As said though in a domestic environment it is unlikely to have PFC higher than most MCB ratings.

 

[Andy™]

I did that once with a 40A MCB switched it on with a dead short across L-E

(absent minded spark had left a test lead across some cables!Blushing:O)

Always a good reason to have the test link long enough to be obvious that it is there