Max zs mccbs/ACB TNS/TT Systems

[JBsparky1]

Hi might be a stupid question but somone might have the answers.

TNS systems and TT where the Ze of the installation is a lot higher than a Ze <0.35 of a TNCS system, how do you comply max Zs of a MCCB in a panel board disconnection time of 5s? Most data manufacturers instruction give really low max Zs values which are not achievable?
Is the only way round this to have combined MCCBS with earth leakage or earth leakage relays /shunt trips .
I also read something in the regs book about omission of RCDS for fault protection, use of supplementary bonding. Calculation required and something to do with touch voltage ?
So many EICRS in the past the disconnection time has not been met if manufacturers data found , otherwise I have LIM it if not data.

 

[davesparks]

TNS systems and TT where the Ze of the installation is a lot higher than a Ze <0.35 of a TNCS system, how do you comply max Zs of a MCCB in a panel board disconnection time of 5s? Most data manufacturers instruction give really low max Zs values which are not achievable?

What size supplies are we talking about here? The 0.8/0.35 guidance for TNCS/TNCS is usually only for small supplies, bigger supplies usually have lower limits.

What kind of Ze values are you measuring on these installations?

My experience of installations large enough to require MCCB distribution is that they are often TNS with a very low Ze, usually they have structural steelwork and lightning protection system which once bonded can bring the Zs values down even lower.

Is the only way round this to have combined MCCBS with earth leakage or earth leakage relays /shunt trips .

In some cases the use of fuses rather than MCCBs can be enough if the Zs is not massively high, likewise an increased size CPC may help.
Otherwise yes you would need to be looking at adding earth leakage protection to the MCCBs.

I don't think it's very common to have a TT supply to larger installations, but if there is then I don't think there's much option other than earth leakage protection.

I also read something in the regs book about omission of RCDS for fault protection, use of supplementary bonding. Calculation required and something to do with touch voltage ?

I think you may be confusing some of the rules for special locations with those for fault protection. Whereabouts in the regs book did you read this?

So many EICRS in the past the disconnection time has not been met if manufacturers data found , otherwise I have LIM it if not data.

Surely that should be an FI and not a limitation. A limitation is for something which has been agreed will not be inspected or tested, in this case you have inspected it and found that more information is required so it requires further investigation.

 

[pc1966]

There are some questions about quite what the system is, are some buildings on TT but the incoming supply is TN-S?

It is unusual, though occasionally possible, for TT to get a low enough Zs to disconnect on the OCPD side as @davesparks has said, but I suspect on a typical MCCB sized to feed other DB, etc, that is not likely.

In such cases you really don't have much option other than an EL add-on to the MCCB, but that is something that adds complexity down-stream as then all other circuits need appropriately set RCD/RCBO protection to achieve selectivity, as those circuits are also unlikely to disconnect on over-current (presumably MCB) side before the EL fires.

If you have something like a TN supplied MCCB panel feeding a sub-DB in another building that is on TT earthing then you need the Zs on the feed cable to achieve the 5s disconnection time. Once in the sub-DB then the RCD protection there (maybe delay RCD incomer, maybe RCBO or MCB+RCD on out going circuits, etc) has to meet the 1s/0.2s disconnection times for TT earthed distribution/final circuits.

A diagram might help us!

 

[JBsparky1]

There are some questions about quite what the system is, are some buildings on TT but the incoming supply is TN-S?

It is unusual, though occasionally possible, for TT to get a low enough Zs to disconnect on the OCPD side as @davesparks has said, but I suspect on a typical MCCB sized to feed other DB, etc, that is not likely.

In such cases you really don't have much option other than an EL add-on to the MCCB, but that is something that adds complexity down-stream as then all other circuits need appropriately set RCD/RCBO protection to achieve selectivity, as those circuits are also unlikely to disconnect on over-current (presumably MCB) side before the EL fires.

If you have something like a TN supplied MCCB panel feeding a sub-DB in another building that is on TT earthing then you need the Zs on the feed cable to achieve the 5s disconnection time. Once in the sub-DB then the RCD protection there (maybe delay RCD incomer, maybe RCBO or MCB+RCD on out going circuits, etc) has to meet the 1s/0.2s disconnection times for TT earthed distribution/final circuits.

A diagram might help us!

Hi all ,

I’m referring mainly to panel boards where MCCBS are fitted and earthing arrangement being TNS . Even with lightning protection and structural steel in some cases the max Zs for distribution circuits has not complied .

I have come across before many times , previous EICRS passed or LIM max Zs on the schedule of test results . Some have had earth leakage devices fitted however other installs clearly don’t meet , once manufacturers information gathered on the zs disconnection times .

The max Zs for typical Schneider mccbs can be reallly low from 100-250a mccbs and in some cases running a larger CSA CPC or even cable itself may still not comply. Especially if say installation Ze is 0.40 however the max Zs for 5s disconnection of the breaker may be 0.25 . Thus only way of then complying being an earth leakage add on kit .

May be a question for my next annual assessment with NIC

 

[pc1966]

I’m referring mainly to panel boards where MCCBS are fitted and earthing arrangement being TNS . Even with lightning protection and structural steel in some cases the max Zs for distribution circuits has not complied .

Generally speaking local earthing will rarely be below a couple of ohms, so in parallel with a TN-S sub-ohm value will make little difference. Helps with lightning of course!

I have come across before many times , previous EICRS passed or LIM max Zs on the schedule of test results . Some have had earth leakage devices fitted however other installs clearly don’t meet , once manufacturers information gathered on the zs disconnection times .

The max Zs for typical Schneider mccbs can be reallly low from 100-250a mccbs and in some cases running a larger CSA CPC or even cable itself may still not comply. Especially if say installation Ze is 0.40 however the max Zs for 5s disconnection of the breaker may be 0.25 . Thus only way of then complying being an earth leakage add on kit .

If your Ze is above the 5s trip impedance then you have a big problem, and an EL kit is one obvious solution.

Depending on the situation (i.e. TN earth, not TT) you might be OK to set the trip current to, say, 10A and trip time to 2-3s so many down-stream MCB would fire before the EL lumbers towards its disconnection, but that is not guaranteeing selectivity as you would get if down-stream final circuits, etc, were all on RCBOs, nor will it always deal with a N-E fault.

In some cases you might get the DNO to convert to TN-C-S as a route to lowering the Ze to a value commensurate with the disconnection times of distribution MCCB that are a modest proportion of the rated supply current (as even 10% regulation means a fault current 10 times that of the rated current, etc). However, just like the EL option that opens a can of worms. Not RCD selectivity worms, but the issue of making sure that all bonds (and sub-main CPC) meet PME sizes. Uprating bonds can be easy, but for buried cables between buildings you might have a problem as the armour may not meet the minimum CSA needed.

May be a question for my next annual assessment with NIC

We would be interested to hear what advice you get.