Zs complies?

[DT1991]

Hi All I'm new to this forum, and is my first question?

So he's the situation.

TNS supply - Ze measured - 0.41ohms

Main switch is RCCB , 100A 100mA

Main switch supplies a DB which feeds another DB ( this is the DB I am interested in) , the cable feeding this is a 2 core SWA, now I come to take a Zdb and the reading I receive is 4.7ohms. This DB is only 15-20 meters away so this instantly throws me out. Tested cable and cable is fine, resistance of the armouring is high but this is expected, not high to the point of there's a clear break in the SWA. Now out of interest I put in a tempory earth to essentially make a 3 core cable and have a dedicated earth. This reading gave back 0.50ohms as Zdb now this reading seems more acceptable.

Now lets go back to the main switch, it's a 100mA RCD so a touch potential of 50v can be applied to ohms law calculated. 50 divided by 0.1 is 500. So I would be allowed 500ohms at any point of the installation? Is this correct I have waffled abit so I might have missed something.

This is the first time i have come across this and I am just seeking answers thanks.

 

[Silly Sausage]

The resistance of the armour shouldn't be high at all.
You must have a problem with the glands or connections on them there glands.

 

[telectrix]

i'd be looking at why the resistance of the armour is high. bad glanding??? you are correct with the value of 500 ohms, but you should not have to rely on a RCD for fault protection.

 

[DT1991]

Forgot to mention I reglanded both ends. And still was getting same readings. This was installed to 15th edition and was tested recently in 2010. Any ideas to what the problem could be? And does the RCD cover the high Zs readings? Thanks

 

[Flanders]

Check continuity between earth bar and SWA at each end and check continuity of SWA it sounds like the SWA is not continuous or not connected and you are picking up a parallel earth feed from some were els

 

[DT1991]

Check continuity between earth bar and SWA at each end and check continuity of SWA it sounds like the SWA is not continuous or not connected and you are picking up a parallel earth feed from some were els

Both ends were not connected with banjos so the earth path wouldn't be taking the SWA route anyway could it, surly just the connection of the gland to the metal casing of both ends is not enough to ensure a connection the earth. I suggested taking the old cable out and running a new 3 core anyway, this seems the best idea.

 

[Silly Sausage]

Both ends were not connected with banjos so the earth path wouldn't be taking the SWA route anyway could it, surly just the connection of the gland to the metal casing of both ends is not enough to ensure a connection the earth. I suggested taking the old cable out and running a new 3 core anyway, this seems the best idea.

In theory, yes.
Get them banjos on and connect them to the earth bars!!!

 

[telectrix]

or buy some earthing nuts.

 

[Flanders]

Would it not be easier and cheaper to fit a banjo and earth link on each end ?
And yes you will get some continuity down the SWA If the glands are tight you would normally get good continuity

 

[DT1991]

In theory, yes.
Get them banjos on and connect them to the earth bars!!!

Did that aswell and was still getting the same reading.

The problem must be the SWA then. I'm just not familiar with 2 core and then SWA been used as the CPC. Lack of experience maybe but I want to know what the solution is. Has is ever been common practice to run a seperate CPC to a DB and just use the armourings as macanical protection?

 

[Engineer54]

Forgot to mention I reglanded both ends. And still was getting same readings. This was installed to 15th edition and was tested recently in 2010. Any ideas to what the problem could be? And does the RCD cover the high Zs readings? Thanks

Unless you have a problem between the earth conductors at each end of this SWA, then i can't say much for your glanding skills... No way should you see a 4+ ohm increase in just 15 to 20 m. Doesn't matter whether an RCD will theoretically cover this high Zs or not, RCD's should be considered as additional protection, not a supplementary protection to get an electrician off the hook. So what happens if this RCD ever failed. The bottom line is you have a problem that needs to be sorted, it's fixable, so fix it!!

 

[Silly Sausage]

Have you fully tested this cable with both ends disconnected?
Get your wander lead out.

 

[DT1991]

Unless you have a problem between the earth conductors at each end of this SWA, then i can't say much for your glanding skills... No way should you see a 4+ ohm increase in just 15 to 20 m. Doesn't matter whether an RCD will theoretically cover this high Zs or not, RCD's should be considered as additional protection, not a supplementary protection to get an electrician off the hook. So what happens if this RCD ever failed. The bottom line is you have a problem that needs to be sorted, it's fixable, so fix it!!

I understand fully. You have to understand in no way am I looking for a get out clause. I am furthering my knowledge.

Simple as that.

 

[Flanders]

Did that aswell and was still getting the same reading.

The problem must be the SWA then. I'm just not familiar with 2 core and then SWA been used as the CPC. Lack of experience maybe but I want to know what the solution is. Has is ever been common practice to run a seperate CPC to a DB and just use the armourings as macanical protection?

Even if you run a separate earth the The armouring needs a good earth connection in case a fault from the live to the armoured occurs.

2 core SWA is common and use to be used all the time but 3 core is sometimes cheaper these days

 

[Engineer54]

Okay, that's fine then....

Well i suggest you re-gland up the wire armouring on both ends of this SWA cable to ensure grip and tightness and then install earthing nut's or banjo's to each gland with fly leads to earth bars. By the way, what size is this SWA cable we are talking about??