Why zs cannot be too big?

[bluesky]

I still dont quite understand why zs cannot be too big? as it explains if zs is too big then RCD wont work properly, but why is that?

cheers

 

[stef]

Zs is less or equal to Uo/Ia and Ze = Zs + (R1 + R2)

 

[Deleted member 9648]

The Zs determines the fault current that will flow if a L-E fault occours at the point where the Zs is measured. Apply simple ohms law ...ie a Zs of 1 ohm on a 230v system will result in a fault current of 230 amps.The fault current that flows determines how quickly the protective device will operate. Thus the Zs must be low enough for sufficient fault current to flow to trip the device in the required time.

 

[ian.settle1]

The lower the Zs then the higher the fault current, the higher the fault current the faster the protective device will operate.

 

[Gravid]

What if your Zs is too high yet your trip times comply?
For instance say on a consumer unit upgrade, and what can be done to rectify this?

 

[JulianC]

What if your Zs is too high yet your trip times comply?

I think the point is that if Zs is higher than the stated value for a particular protective device, the trip time won't comply - there is a direct correlation between trip time and Zs

 

[ian.settle1]

Zs is less or equal to Uo/Ia and Ze = Zs + (R1 + R2)

Ze = Zs + (R1 + R2) Incorrect

Zs = Ze + (R1 + R2) therefore

Ze = Zs - (R1 + R2)

 

[mattyb129]

The lower the Zs then the higher the fault current, the higher the fault current the faster the protective device will operate.

i still belive that a mechanical earth ie conduite trunking is a better fault path than 2.5 4mm cpc on lighting and power circuts it was ok 20 years ago and i still get lower readings wen i test it

 

[ian.settle1]

i still belive that a mechanical earth ie conduite trunking is a better fault path than 2.5 4mm cpc on lighting and power circuts it was ok 20 years ago and i still get lower readings wen i test it

Probably because you are getting parallel paths

 

[mattyb129]

Probably because you are getting parallel paths

i always test both any wayc pc and mechanical i disconnect all metal work fly leads etc take a reading on the cpc then re connect fly leads metal work and test again so wat is the point in running cpcs in a conduite trunking insatalltion

 

[Sintra]

An RCD will trip out regardless of Zs as it is operated by earth leagage therfore if Zs on a circuit is to high then the circuit can be protected by a RCD eg TT system.

On a circuit that is not RCD protected then Zs must be low enough for circuit disconnection in specified time.

 

[pushrod]

What if your Zs is too high yet your trip times comply?
For instance say on a consumer unit upgrade, and what can be done to rectify this?

Your Zs value has to be low enough so that the MCB trips within the correct time in the event of a short circuit. (It is not really connected with an RCD tripping in the event of an earth fault and the trip times you get when rcd testing) If a Zs is too high then you do not get a high enough PFC to operate the overcurrent device (fuse/mcb) within the correct time. You can look those values up in appendix 2 of the OSG. However if your Zs is too low then the PFC could be so big as to cause you to have to upgrade to components with a higher breaking capacity.

edit : had not realised when writing Sintra has already addressed part of my answer (was on page 2)

 

[Deleted member 9648]

i always test both any wayc pc and mechanical i disconnect all metal work fly leads etc take a reading on the cpc then re connect fly leads metal work and test again so wat is the point in running cpcs in a conduite trunking insatalltion

The point is that steel conduit and trunking is liable to corrosion at joints and is very dependant on a high standard of workmanship for good electrical continuity. Thus in the long term a mechanical cpc may be subject to a deterioration in the continuity.

 

[andyb]

The point is that steel conduit and trunking is liable to corrosion at joints and is very dependant on a high standard of workmanship for good electrical continuity. Thus in the long term a mechanical cpc may be subject to a deterioration in the continuity.

Which is why you carry out PIRs.

If you rely on a seperate cpc in steel conduit how do you test it?
In theory you need to drop every socket & light on that circuit to seperate the cpc from the conduit otherwise you will pick up parallel pathes through the conduit.
So if you had a break in the cpc you would not pick this up as you would get a reading through the conduit.

If however you were relying on the conduit then all you would need to do is a long lead test.

 

[mattyb129]

The point is that steel conduit and trunking is liable to corrosion at joints and is very dependant on a high standard of workmanship for good electrical continuity. Thus in the long term a mechanical cpc may be subject to a deterioration in the continuity.

well thats why its all galvanised so it does not deteriorate over time

 

[ian.settle1]

well thats why its all galvanised so it does not deteriorate over time

It is only the coating that is galvanised, when you cut the conduit and then thread it you are exposing it to future corrosion.That is why you are supposed to paint the joint with galvofloid paint after.

 

[Deleted member 9648]

Which is why you carry out PIRs.

If you rely on a seperate cpc in steel conduit how do you test it?
In theory you need to drop every socket & light on that circuit to seperate the cpc from the conduit otherwise you will pick up parallel pathes through the conduit.
So if you had a break in the cpc you would not pick this up as you would get a reading through the conduit.

If however you were relying on the conduit then all you would need to do is a long lead test.

But there is no guarantee that a PIR will be carried out....the reason why CPC's started to be installed in mechanical systems was simply to ensure that continuity is maintained in the event of corrosion. I guess in theory you are correct about separating the cpc and mechanical for testing...but in practice does it really matter?? as long as there is a low resistance R1R2 to the furthest point on the circuit I'm not gonna lose any sleep.

 

[mattyb129]

It is only the coating that is galvanised, when you cut the conduit and then thread it you are exposing it to future corrosion.That is why you are supposed to paint the joint with galvofloid paint after.

the only time ive ever been asked to paint joints is on the london underground and one hospital i worked in but i always make a point of painting weather im asked to or not

 

[zone77]

Your Zs value has to be low enough so that the MCB trips within the correct time in the event of a short circuit. (It is not really connected with an RCD tripping in the event of an earth fault and the trip times you get when rcd testing) If a Zs is too high then you do not get a high enough PFC to operate the overcurrent device (fuse/mcb) within the correct time. You can look those values up in appendix 2 of the OSG. However if your Zs is too low then the PFC could be so big as to cause you to have to upgrade to components with a higher breaking capacity.

edit : had not realised when writing Sintra has already addressed part of my answer (was on page 2)

there would be no need to upgrade components in a domestic installation as main fuse covers this eventuality

 

[pushrod]

there would be no need to upgrade components in a domestic installation as main fuse covers this eventuality

I'm obviously no expert but think you might be wrong in this case if i have under stood things properly.
If Zs was less than 0.038ohms (probably pretty unlikely) then the PFC would be greater than 6kA for a fraction of a second. The mcb of that circuit would then trip. However, for that fraction of a second the huge current could be expected to damage any components that had a lower breaking capacity. If for example the breaking capacity of the mcb was 10 kA you would expect it to survive the dead short and you'd be able to reset it, but anything with a 6kA capacity would be damaged and need replacing.
I didn't think the main fuse comes into it.
Would welcome any other opinions as to whether i am right or wrong .

 

[Deleted member 9648]

I recently did a PIR on a school with some old crabtree C50 mcb's with a 3ka rating and a measured PSC of over 5ka...I attributed a code 2 but our NIC inspector said as long as the rating of the enclosure exceeded the measured PSC it was acceptable.

 

[pushrod]

I recently did a PIR on a school with some old crabtree C50 mcb's with a 3ka rating and a measured PSC of over 5ka...I attributed a code 2 but our NIC inspector said as long as the rating of the enclosure exceeded the measured PSC it was acceptable.

Cheers wirepuller - In other words are they saying that in the event of a dead short those mcbs might be wrecked but because they are in an enclosure with a higher rating that someone standing close by would be safe?

 

[ian.settle1]

Cheers wirepuller - In other words are they saying that in the event of a dead short those mcbs might be wrecked but because they are in an enclosure with a higher rating that someone standing close by would be safe?

Do you know if ordinary switches in a lighting circuit are given a short circuit withstand capacity or is that info hidden away inside the BS-EN number?

Agree it is not the enclosure that is protecting the circuit but the CPD.

 

[bugsy]

Just good practice, over a period of time joints and couplers can become corroded (not so much with galv.)

 

[Deleted member 9648]

Cheers wirepuller - In other words are they saying that in the event of a dead short those mcbs might be wrecked but because they are in an enclosure with a higher rating that someone standing close by would be safe?

Hi Mate,
Thats the gist of it....but I dont buy it....to my way of thinking if a 3ka mcb may be subjected to over 5ka that is not acceptable.....we changed the affected DB's for 10ka rated mcb's.

 

[Davey101]

Whilst we are on the subject, has anyone seen an mcb destroyed, or damaged by too high a PFC?

 

[pushrod]

Whilst we are on the subject, has anyone seen an mcb destroyed, or damaged by too high a PFC?

I haven't started working yet mate, but no doubt i'll see some once i start

 

[scotsparky]

i am suprised nobody has also mentioned Zs x Idelta n < 50v

this is so in the event of a fault to earth the axtranious conductive parts have a fault voltage less than 50v

 

[pevvers]

I recently did a PIR on a school with some old crabtree C50 mcb's with a 3ka rating and a measured PSC of over 5ka...

Have a look at paragraph 3 of 536.1 in the BRB. This is where the allowance of lower breaking capacaties upstream are discussed, with reference to higher ones downstream.