External earth loop Ze with switched fuse before CU

[Gavin John Hyde]

Got a job where im having to do the usual testing.

I have never tested a set up where due to the tails needing to being so long to reach the consumer unit there has been a switched fuse, this is i believe a standard directive from the DNO the tails then terminate into swa and run from the communal meter cupboard in the corridor and then above fire proof ceiling tiles to the actual flat through the wall into the CU.

Set up is DNO head > meter > isolator > switched fuse switch > into SWA> run via ceiling cavity in hall way> enters CU in flat.

There is a separate earth cable from the DNO service head that doesn't terminate until it reaches the CU in the flat.

I have come across them but never done the testing myself when a switched fuse has been present.

When doing the Ze test normally you disconnect main earth from the supply and measure between line and disconnected earth to get Ze.

Given the presence on the fused switch, will it affect the results or testing procedure? As when comparing it to the typical max values from western power it kind of helps to know if what you got is a correctly obtained test result.

I have looked at guidance note 3 but not found a clear answer.

Should i test for Ze between the meter and fused switch out in the communal cupboard rather than the normal test at the consumer unit?. I will almost certainly have to use a wander lead to go from the flat to the cupboard to connect to earth.

It is just a case of where should i connect to line for the Ze test?

My view is if we really are testing at the origin then it would be best to test before the switched fuse.

any thoughts or advice?

 

[Leesparkykent]

Ze and PFC should be done at the origin. Ze with earth disconnected due to parallel paths and PFC with it connected as the lower the impedance the higher the fault current. Zdb then done at the flats consumer unit.

 

[Flanders]

I would test at the switched fuse then treat the db as a sub circuit.

 

[Pete999]

Got a job where im having to do the usual testing.

I have never tested a set up where due to the tails needing to being so long to reach the consumer unit there has been a switched fuse, this is i believe a standard directive from the DNO the tails then terminate into swa and run from the communal meter cupboard in the corridor and then above fire proof ceiling tiles to the actual flat through the wall into the CU.

Set up is DNO head > meter > isolator > switched fuse switch > into SWA> run via ceiling cavity in hall way> enters CU in flat.

There is a separate earth cable from the DNO service head that doesn't terminate until it reaches the CU in the flat.

I have come across them but never done the testing myself when a switched fuse has been present.

When doing the Ze test normally you disconnect main earth from the supply and measure between line and disconnected earth to get Ze.

Given the presence on the fused switch, will it affect the results or testing procedure? As when comparing it to the typical max values from western power it kind of helps to know if what you got is a correctly obtained test result.

I have looked at guidance note 3 but not found a clear answer.

Should i test for Ze between the meter and fused switch out in the communal cupboard rather than the normal test at the consumer unit?. I will almost certainly have to use a wander lead to go from the flat to the cupboard to connect to earth.

It is just a case of where should i connect to line for the Ze test?

My view is if we really are testing at the origin then it would be best to test before the switched fuse.

any thoughts or advice?

Test fro the tail connection at the CU and note it as ZE@DB

 

[7029 dave]

If you decide to test at the s/fuse,remember to keep all earth leads connected/ parallel paths Zdb is tested this way.How will you test Ze without pulling the main fuse and disconnecting the earth lead.?

 

[Gavin John Hyde]

If you decide to test at the s/fuse,remember to keep all earth leads connected/ parallel paths Zdb is tested this way.How will you test Ze without pulling the main fuse and disconnecting the earth lead.?

the set up has a double pole isolator immediately after the meter before the switched fuse i can use to isolate the system to safely access the main supply tail in the switched fuse enclosure. the main earth is locked into the dno head and enclosure( there may or may not be a bar - but it has 2 seals) and not accessible until it enters the Cu in the flat so it looks like a job for the wander lead that is somewhere in my van.

 

[davesparks]

It sounds to me like you cannot test Ze due to the incoming earth connection being inaccessible.

Testing from the far end of the earthing conductor you have won't give you Ze, it will Ze + the impedance of the cable, which is effectively Zs of the sub-main in your setup

 

[Ian1981]

Bare in mind the regulations state a protective conductor should be ran to each point or accessory so I'd class the switch fuse(guessing its plastic?) as a point.

 

[Gavin John Hyde]

Well i am going to test it at the tails entry terminals immediately before the switched fuse and use the main earth cable before it enters the CU. the earth lead in question is approx 14 metres long so will have its own noticeable impedance. i will note it on the test schedule as a limitation. beyond breaking the second seal on the DNO there is no clear solution.

 

[davesparks]

So you will be getting your Ze by enquiry instead of testing it I presume?

Testing at the far end of the long earthing conductor will not give you Ze, it will be Ze plus the resistance of the earthing conductor.