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.

 

[dokkan1080]

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.

So if your voltage drop was complaint but your zs was not using a rcd or rcbo would make the circuit comply?

But I'm guessing that would be bad design but still complaint....:/? Of so why would it be done like that would it be down to cost or lack of a better option?

 

[pc1966]

So if your voltage drop was complaint but your zs was not using a rcd or rcbo would make the circuit comply?

Yes, it would then comply.

The quickest way to determine what is going to be OK is to look at the IET's On-Site Guide in Table 7.1(i) that lists the most common combinations of cable and over-current protection for:

• Ring final circuit (5% drop, loads distributed)
• Lighting radial circuit (3% drop, loads distributed)
• General radial (5% drop, load at end of circuit)

In the columns for max length if has 'typical' cases for Ze for TN-S and TN-C-S cases, and with/without RCD protection. It also tells you what is limiting the length where they vary (e.g. volt drop, Zs, or not permitted as adiabatic limits exceeded, etc).

It also advises on the typical "installation methods" that are acceptable, but does not cover other thermal derating requirements due to bundled cables or hot zones. You need to consider them separately in the design stage.

But I'm guessing that would be bad design but still complaint....:/? Of so why would it be done like that would it be down to cost or lack of a better option?

It is not bad design as such.

But it is not the best design as you then depend on the RCD electronics to protect against even a hard fault (not just direct-contact electric shock) and the reality is in most cases folk do not periodically test RCDs. Even though CU are polluted with labels for all sorts of pointless things including this one important aspect!

The thermal/magnetic trip of the MCB side of a RCBO (or in series with a RCD) is much simpler and more reliable, but depends on Zs being low enough.

So my own advice would be in cases such as this you need to look at the risks and justify if a single RCD is an acceptable trade off.

Where it is likely is something like a small fixed load in a remote location, say a couple of floodlights, where the design current is only, say, 1-2A and thus a long run of 1.5mm or so is perfectly OK, but then Zs is too high for your usual 6A B-curve MCB or similar. You can then look at:

• Is the circuit high risk? If it is run well out of normal reach then the consequences of a failed RCD are less of an issue than if it is, say, a floodlight fixed on something like a hand railing, etc.
• Can I fit a smaller MCB? If you are using RCBO, and for many domestic CU choices, then 6A B-curve is the smallest (in the sense of highest Zs), but for TPN style boards for commercial/industrial you may get MCB down to 2A C-curve (with Zs requierment equivalent to a 4A B-curve) or less. E.g. Hager go down to 0.5A C-curve for the likes of a bell transformer, etc.
• Should I use a bigger cable? Going from 1.5mm to 2.5mm might not add too much cost anyway, but if you are going from 25mm SWA to 50mm it is a serious consideration for cost and wrangling!
• Dual RCD if high risk or TT? Having a 100mA or 300mA delay RCD as the DB incomer means you have selectivity with a working 30mA RCD/RCBO, but should it fail the incomer will disconnect everything. Yes, there is a risk there, but if significant then you ought to have emergency lighting, etc anyway as power can go off for reasons other than your 1.5mm & related RCBO circuit failing!

 

[dokkan1080]

Yes, it would then comply.

The quickest way to determine what is going to be OK is to look at the IET's On-Site Guide in Table 7.1(i) that lists the most common combinations of cable and over-current protection for:

• Ring final circuit (5% drop, loads distributed)
• Lighting radial circuit (3% drop, loads distributed)
• General radial (5% drop, load at end of circuit)

In the columns for max length if has 'typical' cases for Ze for TN-S and TN-C-S cases, and with/without RCD protection. It also tells you what is limiting the length where they vary (e.g. volt drop, Zs, or not permitted as adiabatic limits exceeded, etc).

It also advises on the typical "installation methods" that are acceptable, but does not cover other thermal derating requirements due to bundled cables or hot zones. You need to consider them separately in the design stage.


It is not bad design as such.

But it is not the best design as you then depend on the RCD electronics to protect against even a hard fault (not just direct-contact electric shock) and the reality is in most cases folk do not periodically test RCDs. Even though CU are polluted with labels for all sorts of pointless things including this one important aspect!

The thermal/magnetic trip of the MCB side of a RCBO (or in series with a RCD) is much simpler and more reliable, but depends on Zs being low enough.

So my own advice would be in cases such as this you need to look at the risks and justify if a single RCD is an acceptable trade off.

Where it is likely is something like a small fixed load in a remote location, say a couple of floodlights, where the design current is only, say, 1-2A and thus a long run of 1.5mm or so is perfectly OK, but then Zs is too high for your usual 6A B-curve MCB or similar. You can then look at:

• Is the circuit high risk? If it is run well out of normal reach then the consequences of a failed RCD are less of an issue than if it is, say, a floodlight fixed on something like a hand railing, etc.
• Can I fit a smaller MCB? If you are using RCBO, and for many domestic CU choices, then 6A B-curve is the smallest (in the sense of highest Zs), but for TPN style boards for commercial/industrial you may get MCB down to 2A C-curve (with Zs requierment equivalent to a 4A B-curve) or less. E.g. Hager go down to 0.5A C-curve for the likes of a bell transformer, etc.
• Should I use a bigger cable? Going from 1.5mm to 2.5mm might not add too much cost anyway, but if you are going from 25mm SWA to 50mm it is a serious consideration for cost and wrangling!
• Dual RCD if high risk or TT? Having a 100mA or 300mA delay RCD as the DB incomer means you have selectivity with a working 30mA RCD/RCBO, but should it fail the incomer will disconnect everything. Yes, there is a risk there, but if significant then you ought to have emergency lighting, etc anyway as power can go off for reasons other than your 1.5mm & related RCBO circuit failing!

My dude you are a legend real, the above is like a college lesson.

So what if its out of reach like a light and a class 2 fitting then its not high risk...

But a metal fitting light switch is...

The fact you can go lower than 6 amp mcb is pretty cool that means you can use longer circuit lengths and still meet VD and or zs more easily but how would you find the max zs of breakers lower than 6 amps?

 

[pc1966]

My dude you are a legend real, the above is like a college lesson.

Thanks, hope it helps!

So what if its out of reach like a light and a class 2 fitting then its not high risk...

But a metal fitting light switch is...

Also depends on the risks. You can't normally "grab" a light switch so the risk of injury from shock is only high if you could also be well grounded at the time. Are there other metal objects the operator could simultaneously touch? Is it a wet zone or outdoors?

Failing that, change to a plastic switch.
Or see it it can have heavier wire feeding that, or is otherwise earthed anyway.
[automerge]1598092218[/automerge]

The fact you can go lower than 6 amp mcb is pretty cool that means you can use longer circuit lengths and still meet VD and or zs more easily but how would you find the max zs of breakers lower than 6 amps?

For any device (MCB, fuse, MCCB, etc) if you look at the manufacturer's data it will show curves of time and current. You look along the time = 0.4s axis and see what (maximum) curve is that hits that, and down to find current is needed to reach there. Then you compute the Zs from:

Zs = 0.8 * (0.95 * 230) / Itrip

The 0.95 factor allows for minimum supply voltage and the 0.8 factor is for cables heating up. Though if you are running a cable well below 70C due to a small MCB relative to the usual current carrying capacity that might not be needed.
[automerge]1598092661[/automerge]
In fact for standard MCB is it even easier as the requirements for "instantaneous" tripping (actually 20ms or a bit less) also are the ones that will give you less than 0.4s and they are:

• B = 3-5 * In
• C = 5-10 * In
• D = 10-20 * In

So for an example of a 2A C-curve MCB you have Itrip = 10 * 2 = 20A (max) and then:

Zs = 0.8 * (0.95 * 230) / 20 = 8.74 ohms

 

[pc1966]

One final thing to consider for unusually long cables are:

• If you are failing to meet the Zs for MCB disconnections times, or
• If you fail to meet 5% voltage drop at the MCB rating (even if it is OK at the fixed load's current that you intended)

They are both a warning signs that you need to consider the design in far more detail.

Basically you might use an RCD to mitigate the high Zs situation, but you also have to consider the risk of a L-N fault also failing to disconnect quickly (as that is not considered a fault by the RCD). So you need to make sure that the over-current aspect (i.e. MCB) will prevent a fire if the cable is overloaded for a long time due to a end-of-line short not resulting in enough current to trip the MCB quickly.