TN-C-S High Ze

[wanbo]

I recently completed a report on an exsisting Installation - TN-C-S Two 400mm SWA Supply cables with 185mm Earth and 1250A ACB

Reading of Ze was 0.22 ohms. Although not over the maximum permitted it means all Zs readings for submains circuits ARE over the maximum permitted. The smallest submains circuit on site is a 100A MCCB which has a maximum of 0.18 ohms, which is already nearly above the reading of Ze. All other submains are 200A & 315A.

i was quite surprised that the Ze reading was so high given its a TN-C-S and the size of the supply cables. It was however obtained when the factory was in full production and could not be turned off or main earth disconnected. That is the next step to get the true reading of Ze and if above will be the supply companies problem. However if the Ze does increase but not above 0.35 ohms is it acceptable to install rods, increase bonding conductor size to steel etc to reduce the general reading of Ze. I doubt the client will wish to install earth leakage devices at mains ACB and/or submains MCCB and I also don't believe the MCCBs are adjustable

 

[Des 56]

I found informed opinions of a problem much the same posted here
http://www.electriciansforums.net/commercial-industrial-electrical/43361-high-zs-800a-mccb.html
and here
IET Forums - MCCB's and Max Zs

 

[wanbo]

Thanks for the links. Read on one about distance from the transformer too main panel. This installation isn't close to the transformer at all. Gonna have to be a phone made to the supply company I think.

Thanks again

 

[Engineer54]

So you never actually tested Ze then, if you didn't disconnect the main earthing conductor from the MET!!! Your reading values are all Zs...

I've not seen an ACB without adjustable parameter settings, or without Earth Fault facility!! As for installing rods, what are you expecting, from banging in a few rods, when you already have a 0.22 Zs value?? Increasing bonding conductor size isn't going to improve your Ze value is it, seeing as you need to disconnect them from main earthing conductor, in order to read a Ze value!!!

 

[wanbo]

As mentioned in my post NO the main earth was not disconnected as it was a large factory in in full production!!! I also mentioned that I will be going back to get a true reading of Ze when the factory is closed (probably on a sunday). I wasn't talking about the ACB being adjustable.....I was talking about the 100A, 200A & 315A MCCBs which AREN'T Adjustable. . Although it isn't the reading of Ze I'm worried about, as also mentioned in my post, as the ACB is fully adjustable so it isn't a problem, it is the reading of the submains supplies which are above the maximum permitted for their protective devices, because of the high Ze. I was merely asking if "banging" in earth rods and upgrading earth bonds was an ACCEPTABLE method of reducing the GENERAL reading of Ze/Zs to leave the submains with lower readings also to comply with the max zs of their protective MCCBs. Basically if the true reading of Ze is not above 0.35 ohms and the supply authority decide they don't have to do anything about is it acceptable to install earth rods, increase bonding conductor sizes in order to decrease the general (that is with all earths connected at MET) reading of Ze (or Zs if you wish), low enough to leave the Zs readings of the submains circuits below their maximum permitted or is the only option available to install earth leakage devices at the submains MCCBs

 

[johnboy6083]

its is acceptable, and may well be the only way you can acheive discrimination in your installation. Is the steelwork bonded at each DB? Is the amour effectively bonded at each and every point. i supposed the way you can calucltae this, is is the resistance of the R1+Rn, below the Zs threshold of the MCCB? if it is, then by reducing your R2 resistance then you have a shot at getting the Zs to comply. If the R1=Rn alone is higher than the maxpermitted, then you have to see if the R1 alone is lowers than the max Zs. if it is, then you will have to massivley improve your earth path via bonding ect. It may or may not give you the reading you desire, and the only way you will know is by taking Zs measurements with "everything" bonded down.

EDIT, ive just reread the above, and you are already above the threshold for most MCCB's. I suppose the only other way to see if it will comply, is to recalculate your Zs at each pont using actual measered volts. I reckon that this would not yeild any luck though, as if your Ze is high, then the volt drop would probably be high too.

 

[vernam616]

My understanding is that Ze is at the ORIGIN of the installation, anything thereafter such as submains would be a Zs ??

 

[plugsandsparks]

Had a similar problem and was just scraping by, using smaller breakers than the cable could take matching closely to the machines. Supplementary bonding through steelwork etc did not make much difference in my case. Final ccts using fat cable did...lol
Its a real bummer i'm afraid, it could mean you have to use many sub boards to keep final ccts short. .... i would defintely test it when loads are disconncted as you really need to know what you are up against. Good Luck

 

[Engineer54]

My understanding is that Ze is at the ORIGIN of the installation, anything thereafter such as submains would be a Zs ??

Or as they call the Zs at sub boards now, ...Zdb!!! But it's still basically a Zs value your reading... lol!!

 

[malcolmsanford]

I know the OP is not on, but was wondering if that Zs value for the 100amp MCCB was the manufacturers values or the value taken from Table 41.3

The 0.18 value he quotes would be if you use the 0.23 value for an 100amp C type unit and the 0.8 coefficient.

Often manufacturers values are a lot different from the regs, and where you fail in the book, you pass with manufacturers data

 

[Engineer54]

As far as i know, BS 7671 doesn't give MCCB Zs values, as there are so many derivatives for any given MCCB model and rating. As MCCB's and MCB's do not come under the same BS numbers or criteria as each other, they should never be compared like for like. You will always need the manufactures data for any given MCCB's max Zs...

 

[malcolmsanford]

Agree Col the BS EN 60947 family is for MCCBs, I was just though wondering if that is what he did

 

[Engineer54]

Agree Col the BS EN 60947 family is for MCCBs, I was just though wondering if that is what he did

My post was more for the less experienced Malc, not you ...lol!!!

MCB's are one exception where i would not under any circumstances mix different manufactures units in a DB. It's hard enough with a single manufacture complying with max Zs values...lol!!!