First post

[the__apprentice]

Hi

Just a little background about me, Im nearly through my first year at college carrying out an electrical apprenticeship, hopefully someone can answer my question for me:-

We were meant to be installing some emergency lights in FP200 1.5mm today but when we got to site the inhouse electrician / maintenance man said that the Ze value is so low (I think, or was it so high.....) the cable would need to be urgraded to 2.5mm, but isn't the cpc the same size 1.5mm on both cables?

Im assuming there is a formula to calculate the required cable size from the Ze reading for the inhouse electrician to come to this conclusion - what is it?

Why am I asking here and not my boss, basically I dont want to look stupid in front of him!

Regards

 

[Lectrician]

The earth maybe, but the live is not. Remember the earth loop is formed from the live and earth.

I Would be surprised if the earth loop would be too high - what size breaker, and what length of run?!?

Also, why are you wiring EM lights in FP? I am assuming as you are talking about earth loops that it is mains you are wiring? If so, (and also assuming you are not wiring from a central battery reserve or standby genset), then the self contained fittings you are wiring are not required to be fed via FP type cabling, and in doing so you are actually preserving the mains feed to them which will cause them to remain UNilluminated during a fire etc.

 

[the__apprentice]

breaker was a 10amp and before we even looked at the length of run he said the Ze is that bad the cable requirement is a minimum of 2.5mm, you are correct in assuming the lights are 230v and not from a central battery, with regards to the FP I think it was a customer requirement, I assumed it was part of the British Standards that it must be in FP200 but I get your point about during a fire.

Are you able to demonstrate how the Ze was calculate to determine the cable size, I've been shown how to calculate your cable size from the load and applying the de rating factors for temp etc but not from the Ze - maybe this is in another year??

 

[bernardgreen]

If the lights are the type that contiuously illuminated and switch to battery on power failure then FP cable is the better option.

The lights will run on mains until it finally fails at source. The later the batteries take over the longer there is light.

If the lamps are the type that come on only with mains faliure then maybe FP is over kill.

 

[RF Lighting]

The normal reason for upping the cable size due to the length of the run is to counter act volts drop rather than exceeding the permitted Zs

the max permitted Zs for a type B 10A MCB is 3.84 ohms which will allow you a fairly substantial run in 1.5mm²/1.0mm² T&E (Assuming the external Ze is within the allowable limits)

 

[Adam_151]

Are you able to demonstrate how the Ze was calculate to determine the cable size, I've been shown how to calculate your cable size from the load and applying the de rating factors for temp etc but not from the Ze - maybe this is in another year??

Your Zs value must be lower than the max Zs value allowed for the particular protective device from the tables in BS7671 (after taking into account that the cables will be hotter when loaded than when being tested - quickest way is to make sure they are no more than 3/4 of value in bs7671)

Zs is made up of Ze added to R1+R2 where R1+R2 is the sum of the phase and protective conductors, and bigger cable CSA means smaller R1+R2 for given length and conductor material

Also size of protective conductors should either comply with table 54G or be calculated with the adiabatic equation (now that will be a fun one when you get to it! )

 

[the__apprentice]

over kill then, the lights are non maintained. I'd guess normal twin and earth would be much cheaper to install as well, what would happen if you wanted to run some outside, Im told fp has a better mechanical protection so I assume that would be better?

++++++++++++++++++++++++++

Zs is made up of Ze added to R1+R2 where R1+R2 is the sum of the phase and protective conductors, and bigger cable CSA means smaller R1+R2 for given length and conductor material

So would you estimate your length of run to calculate the r1+r2 and apply the Ze?
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Lynda, moderator

please use the edit button

 

[Adam_151]

So would you estimate your length of run to calculate the r1+r2 and apply the Ze?

Yeah, when designing, you might select a protective device, look up max Zs for disconnection time required, take 75% of that, subtract Ze, which tells you what you have left for R1+R2, then you take how long your cable is, look up the resistances for the conductor sizes of the phase and protetcive conductors you want to use and see if it fits, if it does then great, if not, you can play around with the conductor sizes or protective device, or work through the proper correction calculation* rather than using the 75% rule of thumb

(*-not that I entirely agree with making everything just fit with a few decimal places margin though, much better in my mind to design with a bit of a margin, Ze is not perfectly constant for example and can fluctuate, etc)

 

[the__apprentice]

thanks for the great explination