Disconnection times not met

[pc1966]

Thanks for your help guys, yes my only concern or lack of knowledge was the MCCB and the Zs value I got which was exceeding the Max values allowed, now the MCCB has 3 settings 125A, 140A, 160A but I believe even setting on 125A I won’t be achieving the values I was getting,

The simpler MCCB only let you adjust the thermal curve by 20-40% or so. They don't give you control over the "instant" trip that is from the magnetic aspect and it is that which usually gives you the sub-5s disconnection times.

The fancy electronic MCCB has the ultimate fall-back of a instantaneous magnetic trip at some high current (typically 10 or more times the max thermal limit) but they allow you to fiddle with the "thermal curve" as it is done electronically, so you can define constant I2t ranges, etc, according to your cable protection requirements, etc.

 

[Bradley6969]

The simpler MCCB only let you adjust the thermal curve by 20-40% or so. They don't give you control over the "instant" trip that is from the magnetic aspect and it is that which usually gives you the sub-5s disconnection times.

The fancy electronic MCCB has the ultimate fall-back of a instantaneous magnetic trip at some high current (typically 10 or more times the max thermal limit) but they allow you to fiddle with the "thermal curve" as it is done electronically, so you can define constant I2t ranges, etc, according to your cable protection requirements, etc.

Thanks for that

 

[Julie.]

Yes, to add to what @davesparks said - when you have the cable fed from the MCCB it should meet the disconnection time of 5s for the end Zs value just in case there is damage at any point along the cable. My earlier comment was just as he said - for a MCCB incomer.

Changing the MCCB to include RCD capability would be one fix. However if you get one of the fancier MCCB you might be able to configure the over-current trip curve to meet 5s for your cable-end Zs of 0.44 ohm (say setting it to trip at 5s for 0.95 * 230 / 0.44 = 496A) so you meet cable disconnection, your "instant trip" is still well beyond any downstream MCB for selectivity, and you are not having to find space for a MCCB add-on module.

I don't know enough about the details of MCCB selection to say for sure, probably @Julie. or @davesparks would have a lot more knowledge than me for this aspect.
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Me? Talking nonsense on internet forums seem to be my current status.

At least I keep my clothes on for this forum

Firstly, based on the information, I would assign FI - this could be a very serious issue as I highlight below.

In respect of the mccb settings, that would be a no-go, if you take a measured loop of 0.44ohm, if this is measured cold then we need to correct it for hot running, and then 95% voltage, this would mean the mccb would need to trip in under 5s at circ 688A.

Since a typical mccb has the instantaneous around 10x this would mean a mccb of 68A(~63), or achieving 5s in the time-current curve; typically this would be circ 8x the overload setting, so you would need to set it at around 85A - this would be too low for the circuit I would guess.

The real issue though is why such a high impedance?

I don't know the number of cores, but if it's a 4core 70mm2 cable 40m long, I would guess a r1 of 0.011ohm, and a r2 of 0.048 ohm (The armour should be sufficient if it's 4 or more cores - if it's fewer than 4 core though, it wouldn't), anyway this means the Ze (or Zs at the feeder board) is around 0.4 ohm.

For something feeding 160A circuits this appears very high, and means either there is a bad connection, which could end up being c1; or the incoming fuses wouldn't trip in any sensible time. (C2 on the main protection, and on probably almost all other circuits).

This really needs further investigation, hopefully a loose connection and a cheap fix, but if it is a high Ze then the dno would have to sort it out as their own fuses wouldn't operate in time, only then can you investigate what needs to be done, if anything.

 

[davesparks]

So just to confirm it would be a C2 on report? Sorry I’m trying to understand incase I come into this situation again

In normal operation it is not dangerous, so it is not immediately dangerous, so it's not a C1.
This would be dangerous under fault conditions, so it is potentially dangerous, which is C2.
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Firstly, based on the information, I would assign FI - this could be a very serious issue as I highlight below.

Why do you say FI for this?
We know that the Zs is too high, so there is a potential danger which is a clear C2 in my view.
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I don't know enough about the details of MCCB selection to say for sure, probably @Julie. or @davesparks would have a lot more knowledge than me for this aspect.

I'm willing to bet you know more about it than I do.

 

[Julie.]

In normal operation it is not dangerous, so it is not immediately dangerous, so it's not a C1.
This would be dangerous under fault conditions, so it is potentially dangerous, which is C2.
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Why do you say FI for this?
We know that the Zs is too high, so there is a potential danger which is a clear C2 in my view.

Because we don't know if its a poor connection, if it is, then under normal conditions this could be a dangerous - immediate risk hence c1 , but yet if it's only a high resistance somewhere c2 would be the case, it's a little academic as FI, C1, and C2 all need fixing.

 

[davesparks]

Because we don't know if its a poor connection, if it is, then under normal conditions this could be a dangerous - immediate risk hence c1 , but yet if it's only a high resistance somewhere c2 would be the case, it's a little academic as FI, C1, and C1 all need fixing.

No we don't know the cause, but we do know it is potentially dangerous so that is how it should be recorded.
If a loose connection is found, and if that is a C1 item then that would be recorded on the EICR as another item (or more likely just tightened up there and then if it can be isolated)
A loose connection in a live conductor would usually be quite obvious, especially at that kind of current so I doubt that this is the cause.
It could be a loose connection in the CPC but that too is a C2 item

 

[Julie.]

No we don't know the cause, but we do know it is potentially dangerous so that is how it should be recorded.
If a loose connection is found, and if that is a C1 item then that would be recorded on the EICR as another item (or more likely just tightened up there and then if it can be isolated)
A loose connection in a live conductor would usually be quite obvious, especially at that kind of current so I doubt that this is the cause.
It could be a loose connection in the CPC but that too is a C2 item

Agree, but I would use FI as it could be more serious than C2, and this is exactly what FI is there for

 

[davesparks]

Agree, but I would use FI as it could be more serious than C2, and this is exactly what FI is there for

This is not the way I understand the coding, FI is for items which require further investigation to establish whether they are affecting safety or not.
In this case there is a clear potential danger, no further investigation is required to establish this, so its a C2.

 

[bigspark17]

Have you confirmed that the tns supply has not been converted to a tncs? In my view any supply to a caravan site should be TT earthing arrangement with upfront rcd time delay.
How old is the installation?

 

[Julie.]

This is not the way I understand the coding, FI is for items which require further investigation to establish whether they are affecting safety or not.
In this case there is a clear potential danger, no further investigation is required to establish this, so its a C2.

Yes, it is at least a C2, but I would mark it as FI because we don't know whether it is more serious or not.

It has a big potential for being C1

However if I was on-site, I would have got the Zs at the feeder board, so would be able to narrow it down - perhaps the op did, but just hasn't posted this information.

The high figures stand out as something potentially dangerous given the circuit sizes, they are not just a little on the large size.
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Have you confirmed that the tns supply has not been converted to a tncs? In my view any supply to a caravan site should be TT earthing arrangement with upfront rcd time delay.
How old is the installation?

Agreed, I assume it's a fairly remote site fed by wet string, these tend to have high loop impedance, but usually the dno would have taken issue with the lack of earth protection when first connected or upgraded given the current rating.