Disconnection times not met

[Bradley6969]

Three questions haha

1. 160A MCCB the max Zs for this particular switch is 0.25 ohms all tested and I’m getting 0.44 ohms its a 95 SWA outgoing which feeds another panel which has its own outgoing circuits would I be right and say that would be a code 2? As I can’t meet disconnection time on it

2. Can’t meet disconnection times for various circuits 63A MCBs no RCD protection would you code 2? Max Zs should be 0.69 ohms and I’m in the 1.00 ohms

3. If I was to add RCD to question 2 could I achieve ADS - or would I need to test L-N also for my short circuit protection

 

[pc1966]

What sort of PSSC are you seeing relative to the PFC?

Those sort of numbers imply a significantly higher CPC impedance than the neutral or the voltage drop is going to be missed by a significant margin.

If it is down to the SWA armour impedance being too high (or rubbish connection to the glands/banjos), then to get the CPC impedance down a supplementary run of copper conductor along side together with decent connections at the ends might be a simpler/cheaper overall fix than changing to RCDs.

 

[ipf]

Where have you got your maximum Zs rating from?

 

[Bradley6969]

What sort of PSSC are you seeing relative to the PFC?

Those sort of numbers imply a significantly higher CPC impedance than the neutral or the voltage drop is going to be missed by a significant margin.

If it is down to the SWA armour impedance being too high (or rubbish connection to the glands/banjos), then to get the CPC down a supplementary run of copper conductor along side together with decent connections at the ends might be a simpler/cheaper overall fix than changing to RCDs.

Hi, thanks for the reply it’s a small site with some 20 caravan boxes so all SWA cable and underground, I’m just wanting to make sure my coding is right
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Where have you got your maximum Zs rating from?

Manufactures data

 

[pc1966]

Where have you got your maximum Zs rating from?

That is a good point, just checked and a 63A B-curve MCB should be 0.56 ohms from OSG.

MCCB are more of a pain as you need to look up a specific model (and/or what settings are dialled in to it) but scaling a 100A B-curve MCB points to 0.21 ohm as being more likely for a 160A device.
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Manufactures data

Interesting, as Wylex have this for their miniature 63A MCB:

• B-curve 0.729 ohm
• C-curve 0.364 ohm
• D-curve 0.183 ohm

 

[Bradley6969]

Would I be right for saying code 2 for questions 1 and 2? Is it just poor design for TNS system
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That is a good point, just checked and a 63A B-curve MCB should be 0.56 ohms from OSG.

MCCB are more of a pain as you need to look up a specific model (and/or what settings are dialled in to it) but scaling a 100A B-curve MCB points to 0.21 ohm as being more likely for a 160A device.
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Interesting, as Wylex have this for their miniature 63A MCB:

• B-curve 0.729 ohm
• C-curve 0.364 ohm
• D-curve 0.183 ohm

I have an app on my phone, plus I’m on the NIC certs so auto puts a maximum reading in for MCBs etc

 

[pc1966]

Would I be right for saying code 2 for questions 1 and 2? Is it just poor design for TNS system

Yes, that would be right. According to the Best Practice Guide #4 it is C2 for:

Earth fault loop impedance value greater than that required for operation of the protective device within the time prescribed in the version of BS 7671/IEE/IET Wiring Regulations or manufacturers’ published data current at the time of installation

 

[ipf]

You could work out the R1+R2 rating for the cable run and see how it compares with your measured Zs minus Ze. If it's similar it's a good reading and something needs sorting. If not, it could be a bad connection.

 

[pc1966]

I have an app on my phone, plus I’m on the NIC certs so auto puts a maximum reading in for MCBs etc

That is a neat feature!

 

[buzzlightyear]

If it is down to the SWA armour impedance being too high (or rubbish connection to the glands/banjos),

this what i was thinking on the same lines .

 

[Bradley6969]

I was just unsure as the 160A MCCB feeds the panel with 63A MCBs but the Max Zs is higher than the Max Zs of the 160A MCCB thought it was a C2

 

[pc1966]

As a carven park I guess most end circuits have RCDs anyway, so you could deal with the (probably) high R2 by going to delay RCDs and/or a delay RCD add-on to the MCCB, but first it would be worth finding out if it is simply too much cable/not enough armour, or if there is some crappy gland use to be fixed.
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I was just unsure as the 160A MCCB feeds the panel with 63A MCBs but the Max Zs is higher than the Max Zs of the 160A MCCB thought it was a C2

Is it feeding just the internal bus of the panel? I.e. there is no cables longer than 3m coming from the MCCB?

If so then the issue is one of meeting disconnection times on the 63A outgoing circuit MCBs, as the MCCB is only needed for overall overload of the system if (as is likely) the sum of MCBs exceeds the supply capability. I.e. no C2 code on the MCCB as it is not providing earth fault protection anyway.

 

[Bradley6969]

As a carven park I guess most end circuits have RCDs anyway, so you could deal with the (probably) high R2 by going to delay RCDs and/or a delay RCD add-on to the MCCB, but first it would be worth finding out if it is simply too much cable/not enough armour, or if there is some crappy gland use to be fixed.

Yes I spoke to my boss and he’s on about putting a 300mA time delayed RCD inside panel with the MCBs as each box has its own 30mA additional protection as all readings are similar and get higher towards ends of circuits
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Yes I spoke to my boss and he’s on about putting a 300mA time delayed RCD inside panel with the MCBs as each box has its own 30mA additional protection as all readings are similar and get higher towards ends of circuits

But he didn’t say anything about the 160A MCCB feeding the panel as that is 0.20 ohms higher???
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As a carven park I guess most end circuits have RCDs anyway, so you could deal with the (probably) high R2 by going to delay RCDs and/or a delay RCD add-on to the MCCB, but first it would be worth finding out if it is simply too much cable/not enough armour, or if there is some crappy gland use to be fixed.
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Is it feeding just the internal bus of the panel? I.e. there is no cables longer than 3m coming from the MCCB?

If so then the issue is one of meeting disconnection times on the 63A outgoing circuit MCBs, as the MCCB is only needed for overall overload of the system if (as is likely) the sum of MCBs exceeds the supply capability. I.e. no C2 code on the MCCB as it is not providing earth fault protection anyway.

So the 160A MCCB feeds the panel with 95 SWA to some bolts ons that feeds a 100A isolator and 63A MCBs

 

[pc1966]

So the 160A MCCB feeds the panel with 95 SWA to some bolts ons that feeds a 100A isolator and 63A MCBs

I think we are getting to the "we need a diagram" stage of the thread!

If the MCCB is feeding stuff beyond a well-protected cable less than 3m in length then it ought to meet fault to earth disconnection times.

I was thinking of a MCCB incomer to a board: in that case the MCCB need not meet disconnection on its own, as all circuits ought to be met by the outgoing MCBs' protection.

 

[Bradley6969]

I think we are getting to the "we need a diagram" stage of the thread!

If the MCCB is feeding stuff beyond a well-protected cable less than 3m in length then it ought to meet fault to earth disconnection times.

I was thinking of a MCCB incomer to a board: in that case the MCCB need not meet disconnection on its own, as all circuits ought to be met by the outgoing MCBs' protection.

Okkkk,
Yes so 160A MCCB fed from EB tails, 40 metre ish run of 95 SWA to the Panel with all outgoing 63A MCBs haha
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So disconnection only applies to final circuits? Thanks so far not really done much with coding and testing