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

 

[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.
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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.
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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.

 

[dokkan1080]

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.

So if its long best bet is to just use a smaller mcb go from 6amp to a 4amp or a c type to a b type or use a bigger cable

 

[pc1966]

So if its long best bet is to just use a smaller mcb go from 6amp to a 4amp or a c type to a b type or use a bigger cable

That is the simplest / easiest option to avoid much more analysis. If the cable is already in then looking at a smaller MCB (if available for the distribution board) would be the simplest approach.

If designing from scratch then looking at "6A B-curve" as readily available and usually in the van as a spare would point to using a bigger cable, as here going from 1.5mm to 2.5mm is not a big cost factor.

 

[dokkan1080]

That is the simplest / easiest option to avoid much more analysis. If the cable is already in then looking at a smaller MCB (if available for the distribution board) would be the simplest approach.

If designing from scratch then looking at "6A B-curve" as readily available and usually in the van as a spare would point to using a bigger cable, as here going from 1.5mm to 2.5mm is not a big cost factor.

But what would count as an overly long circuit?
100m? 150m?

 

[pc1966]

But what would count as an overly long circuit?
100m? 150m?

One where Zs or VD on the chosen cable CSA, length, and MCB are not being met!

Actual length is immaterial.

Simpler answer is one that is longer than the values giving in the OSG for your situation.
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While an electrician should know how to do all of the calculations, etc, as needed, in practice the whole point of "standard circuits" such as the OSG tables is to avoid the effort and make for a quicker, safer, and cost-effective design process.

You look at the load(s) current requirement, look up a readily available cable/OCPD combination that should be OK for that, and see if your expected length is within the table's values of length & method. If there is nothing else unusual (hot zone, big bundle of cables to need more derating, high inrush currents, manufacturer specific requirements, etc) you don't even need a calculator as it is ready to be fitted!

 

[dokkan1080]

?

So of you was tncs type b 6 amp you are pretty much good up to 90 meter.

But if you was to drop to a 4 amp type b mcb you can go longer but as a rule you know 90 meters will comply at 6 amps

So its best to design a circuit to the breaker size and not your loads...?

 

[pc1966]

If it is not in the OSG table you need to go through the full design process really. But for that you also need to know what choices you have for cable and MCBs, etc, that are actually available to you when trying out combinations to see if they meet all of the goals.

MCB/RCBO in particular have to be approved by the DB/CU manufacturer to be sure they are safe (e.g. line up with the busbar, etc) so you need to check the DB make/model and then see what MCB/RCBO/etc are made for it first.
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Many sparks have one or two brands they always use as they know them, have had no problems in the past, can get them are a reasonable price, and usually can get any spares quickly as needed. So they would have a good idea of what they can get for a new design from memory.

If it is an existing design you generally have to work with whatever DB is installed (unless major upgrades are in order) and then you have to do your research.

 

[davesparks]

So if its long best bet is to just use a smaller mcb go from 6amp to a 4amp or a c type to a b type or use a bigger cable

No, whatever the length of the circuit you calculate what size cable is required for compliance.
You select the correct size and type of MCB for the circuit and calculate the cable size after that.