testing calc question

[volta1]

a 10amp type c has a max measured zs of 1.85,i have a circuit with a measured zs of 2.03 the circuit is 2.5mm (not sure type) and it provides roof lighting also r1+r2 was 2.77ohms. is there any way i can make the zs result acceptable using the design zs from the regs or does this circuit just fail? i could reduce the mcb to a 6amp but dont want to.

 

[nickblake]

there are a few ways to do it 1st check all connections ensure they are tight , high R1 + R2 and Zs could be due to loose connections , also depends on the length of cable run , the other way is to increase the size of the cpc ,and the other option is to install an RCD . first i would have a rough idea on the cable length if for instance the lenght of the cable is 30 meters then you R1+ R2 reading should be around the 0.59 ohms up to about 50 meters it should be around 0.98 ohms so if you work on those values it may give you an idea of what your readings should be but i place my money on a loose connection some where.

details of R1 + R2 values are on table 10.1 of the on site guide

 

[malcolmsanford]

I would check the circuit first, see if you have any loose conections things like that.

Instead of that C type have you thought of using a B if there are not large in rush currents.

If the circuit is wired in singles you could rewire with a larger CPC.

You can also use supplementary bonding to help with the Zs.

The 2.77 ohms does seem high to me, it is either a massive run, and the 2,5mm^ is undersized. My gut instinct here is there is a loose connection somewhere.

 

[volta1]

this is someone else's test sheet that they have deemed acceptable,ive tried to make it acceptable using gn3 but can't,was wondering if can be.or if it just fails as it stands.

 

[nickblake]

It fails as it stands as the Zs exceeds the permited value , we can,t suggest any more than malcome and my self have suggested , there could be an under lying problem ie loose connection as we both have mentioned or iits a masive cable run , the only way to sort it out is to carry out investigations , and would have been what id have put down on a periodic inspection observations sheet : Measureed Zs exceeds permitted limits requires further investigation and it would have been given a code 3 ,requires further investigation

 

[solo2771]

As the others say the circuit needs checking for a loose connection, if it is a loose connection in time it will get worse so it is best by far to find what sounds like a faulty connection.
Paul

 

[Chr!s]

Dont want to offend, did you null your meter?

Chris

 

[volta1]

this test result has been deemed acceptable by a super intelligent engineer.who has used his superior knowledge to calculate the result and deem acceptable using the design maximum zs(2.30)off the top of my head.just want to know if this is possible.lets assume the circuit is very long and there are no loose connections.

 

[malcolmsanford]

The maximum Zs to table 41.3 for a 10amp type C BS 60898 is 2.30 and you have used the rule of thumb to bring that down to 1.84.

Now you have said that they measured the Zs, I assume a live test and it was 2.03 and we can assume that is becasue of all those lovely parallel paths, that is because your R1+R2 was 2.77 and so the Zs is a lot lower than the equation would be.

Now to design this you will need the equation Zs = Ze + (R1+R2) so what do we know

Ze ??
R1+ R2 = 2.77 Do you know the Ze value or more to the point did your engineer

Another design way to find R1+R2 is Mohm/m X length/1000 so do you know the length of the run? or more to the point does your engineer.

 

[Chr!s]

this test result has been deemed acceptable by a super intelligent engineer.who has used his superior knowledge to calculate the result and deem acceptable using the design maximum zs(2.30)off the top of my head.just want to know if this is possible.lets assume the circuit is very long and there are no loose connections.

Manufactures info will differ from the brb, so some leeway possible there.

 

[Chr!s]

this test result has been deemed acceptable by a super intelligent engineer.who has used his superior knowledge to calculate the result and deem acceptable using the design maximum zs(2.30)off the top of my head.just want to know if this is possible.lets assume the circuit is very long and there are no loose connections.

Stick a fuse in down stream, make the mcb a distribution circuit, then the fuse the final circuit

 

[malcolmsanford]

Hold on before we go rewiring the installation let's see if we can find out if there is a problem. Sticking it on a RCBO will overcome all the problems but that is not avoiding the fact that he as a circuit design that is not meeting it's disconnection time.

I don't know if this is a EIC or a PIR as the OP just tells us it a cert, but let's make sure we don't have a problem with the circuit

 

[Chr!s]

I don't know if this is a EIC or a PIR as the OP just tells us it a cert, but let's make sure we don't have a problem with the circuit

this test result has been deemed acceptable by a super intelligent engineer.who has used his superior knowledge to calculate the result and deem acceptable using the design maximum zs(2.30)off the top of my head.just want to know if this is possible.lets assume the circuit is very long and there are no loose connections.

Hold on before we go rewiring the installation let's see if we can find out if there is a problem. Sticking it on a RCBO will overcome all the problems but that is not avoiding the fact that he as a circuit design that is not meeting it's disconnection time.

Thats another way, maybe there is an rcd of some descript upstream of the mcb?