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