Zs and circuit length

[dokkan1080]

if you have the ze and know the length of a circuit can you calculate the zs ?

Ze : 0.24
Circuit length : 100m
1.5mm cable twin

 

[Julie.]

if you have the ze and know the length of a circuit can you calculate the zs ?

Ze : 0.24
Circuit length : 100m
1.5mm cable twin

Yes.

 

[dokkan1080]

Yes.

How would you do it I guess because when know the resistance of the cable

 

[Julie.]

How would you do it I guess because when know the resistance of the cable

This is just standard , one aspect of sizing the cable, I would use the cable resistance from the osg or gn3 or of course manufacturers data, this gives ohm per km for R1+R2 , so length x resistance.

You should also do this calculation before testing, so you can confirm that the measured value is as expected

 

[davesparks]

if you have the ze and know the length of a circuit can you calculate the zs ?

Ze : 0.24
Circuit length : 100m
1.5mm cable twin

You can predict the Zs by calculation, but this will not necessarily be the actual Zs of teh circuit.

You can look up the R1+R2 per metre from data sheets to calculate the predicted R1+R2 then add this to the Ze to predict Zs.

This is an important step in the process of designing a circuit.

 

[dokkan1080]

You can predict the Zs by calculation, but this will not necessarily be the actual Zs of teh circuit.

You can look up the R1+R2 per metre from data sheets to calculate the predicted R1+R2 then add this to the Ze to predict Zs.

This is an important step in the process of designing a circuit.

Will the zs be lower or higher or does it depend?

So for example how would I work out the calculated zs of 1.5/1 twin at 100m with a ze of 0.24

Find 1.5mm/1mm in that table times it by 100 then add the ze to it

 

[davesparks]

Will the zs be lower or higher or does it depend?

So for example how would I work out the calculated zs of 1.5/1 twin at 100m with a ze of 0.24

Find 1.5mm/1mm in that table times it by 100 then add the ze to it

Actual Zs should be equal to or lower than the predicted value, within the tolerance of the test equipment used.

Yes, exactly that

 

[Julie.]

Will the zs be lower or higher or does it depend?

Higher or lower than what?

So for example how would I work out the calculated zs of 1.5/1 twin at 100m with a ze of 0.24

Find 1.5mm/1mm in that table times it by 100 then add the ze to it

Yes, but it depends on which table you obtain the data from, some tables aimed at testing (eg in gn3) have the resistance measured when cold - as for testing you want the value similar to that which you would be measuring.

However when designing, you need the resistance when hot/running in order to ensure that the Zs is low enough to operate protection. Similarly hot resistance to determine voltage drop when running.

So either use resistance when hot (for design) and compare against protection limits (also hot) - or use cold values (compare cold resistance against cold protection limits)

The issue is when you have say cold resistances, but hot protection limits, then you must apply a correction so you are always comparing like for like.

You haven't said why you are doing this, there are some tables better suited for testing, and others better suited for the design phase.

 

[ipf]

You should also do this calculation before testing, so you can confirm that the measured value is as expected

Yes, but more for design I'd say.

 

[Julie.]

Yes, but more for design I'd say.

Yes, although given an unfamiliar circuit, I would calculate an expected r1+r2 when measuring it, even if it is within the max Zs of the protection, as if it calculates at 1 ohm, but measures 1.9 (still within the 2.2 of protection) there is still something wrong- perhaps loose connection.

But yes bread and butter calculations at the design stage

 

[dokkan1080]

So if my circuit was 100 meter it would be :
1.5/1 mm cable is 30.20 /1000= 0.0302
0.0302x100= 3.02 + ze wpuld equal zs but the zs should be lower than that due to parallel pathes to earth?

 

[ipf]

Yes, although given an unfamiliar circuit, I would calculate an expected r1+r2 when measuring it, even if it is within the max Zs of the protection, as if it calculates at 1 ohm, but measures 1.9 (still within the 2.2 of protection) there is still something wrong- perhaps loose connection.

But yes bread and butter calculations at the design stage

Yes, we're talking basic stuff.

 

[pc1966]

So if my circuit was 100 meter it would be :
1.5/1 mm cable is 30.20 /1000= 0.0302
0.0302x100= 3.02 + ze would equal zs but the zs should be lower than that due to parallel paths to earth?

Zs might be lower, but it really depends on the type of system.

Usually you find the DB Zs is a bit lower than the supply Ze because of bonding to gas/water pipes, etc, providing parallel earth paths. In your case if Ze went from 0.24 ohm to, say, 0.1 ohm it would make very little difference to the Zs for the 1.5mm circuit as you are adding 3.02 to it. So it has (3.02 + 0.24) / (3.02 + 0.1) = 1.045 (so under 5% change).

What can make a difference is if it is in something like an industrial situation feeding floodlight or similar on a steel structure that are earthed and you have that path in parallel to the T&E's internal 1mm CPC. Then you might see an ohm or so less and that is a significant change in your Zs.

But from a design point of view you should not assume such parallel paths are there, they might not be in the future if something changes (e.g. metal water pipe is replaced by plastic), so you should be designing for sufficiently fast & safe disconnection on a fault from the installed cable & DB value of Zs

 

[dokkan1080]

Zs might be lower, but it really depends on the type of system.

Usually you find the DB Zs is a bit lower than the supply Ze because of bonding to gas/water pipes, etc, providing parallel earth paths. In your case if Ze went from 0.24 ohm to, say, 0.1 ohm it would make very little difference to the Zs for the 1.5mm circuit as you are adding 3.02 to it. So it has (3.02 + 0.24) / (3.02 + 0.1) = 1.045 (so under 5% change).

What can make a difference is if it is in something like an industrial situation feeding floodlight or similar on a steel structure that are earthed and you have that path in parallel to the T&E's internal 1mm CPC. Then you might see an ohm or so less and that is a significant change in your Zs.

But from a design point of view you should not assume such parallel paths are there, they might not be in the future if something changes (e.g. metal water pipe is replaced by plastic), so you should be designing for sufficiently fast & safe disconnection on a fault from the installed cable & DB value of Zs

Cheers man

So if the circuit after all that doesn't meet zs does it comply if put on an rcd or rcbo

 

[pc1966]

Cheers man

So if the circuit after all that doesn't meet zs does it comply if put on an rcd or rcbo

That might be a solution, but generally there is only a small range of cases (i.e. cable length & OCPD rating, etc) when you fail to disconnect on Zs but still are OK for voltage drop. You should check that as well before deciding to go up a cable size or take some other action.

You would not do it for 1.5mm T&E, but for bigger SWA you might find the most cost-effective option is to run a separate copper CPC in parallel with the SWA armour to get R2 down far enough.