Max Zs Value For Tt Distribution Circuit (Not Final Circuit!)

[zak325]

Apologies if this question sounds a bit basic but am a little confused at the mo and quite new to the game. Here goes -

I am trying to complete a NICEIC Highway Installation Certificate for a system where there is a 230 v TT supply coming into a roadside pillar into the main BS88 fused head. 6mm Meter tails then take this into a 30mA RCBO which protects the sole circuit to an accessory.

So on the certificate I have 2 circuits - 1. Supply (Distribution) and 2. Final circuit to accessory (Final)

I know there is some debate on using the 1667 ohm / 200 ohm as max Zs for the protected final circuit figure but what should I use for the max Zs of the actual supply / distribution circuit ?

If i was working on a TN system I could happily just look up the max Zs figures using Table 41.2 but am a bit stumped with this TT scenario.

Any help on this welcomed. Thanks

 

[Andy™]

is the BS88 part separate feeding into an RCBO further down, i.e a complete different circuit? if so, then the max Zs for that part will be that of the BS88 fuse

if its separate but only a short gap between them (i.e next to each other), then your probably going to have to go with 1667, otherwise i doubt you can get a low enough Zs to comply with the BS88

 

[zak325]

hi andy - yes it is separate - sorry if this is a silly question but how / where I can find max Zs values the BS88 fuse ?

 

[steptoe]

I doubt you are going to find the answer in BS7671 for that one,

dunno what regs highway lighting comes under, but Im sure Mr or Mrs streetlighter will know.

 

[Andy™]

strret lighting used to have its own regs in 16th i think. there now gone and as part of the general regs. there is a bit about it in 559.3 but not much

 

[zak325]

how can I contact Mr/Mrs Strretlighter and make them aware of this post?

 

[Andy™]

you could send him a PM

 

[Sidewinder]

It is under 7671, it has just been absorbed into the main regs, though you will also need to look to 7430 (IIRC)

 

[zak325]

sorry but what is 7430 iirc? i have the big red book 7671:2008 at the mo.

 

[binky]

when it comes to TT I look at this way, whilst you can have 1667ohms, I like to see a good circuit as per TN-S values upto rod, then a good rod value. My thinking is that a good circuit will give resistance values similar to house, factory etc etc if you have a 200 ohm rod and 1447 ohms on the circuit, then it says to me that someting is wrong with the circuit. Not necessarily the official answer, but one I can live with. I hasten to add i don't get involved with street lighting so may be I should shut up!!

 

[husky2013]

Need to know a few more details

What Rating is the BS88 Fuse?

What Amperage is the 30mA RCBO?

Are u definate that it is a TT system as it is very unusual for a highway installation to be TT, 99% of the time the the DNO will supply a TNCS service.

Have u seen an electrode pit in the pillar and assumed its a TT system?

have u done a Ze test???

if at all possible could u email me some pictures or even post them on here ill pm u my email address

 

[apprentice87]

Hi all, Why you you even remotely care what value of Zs you need for the BS88 cut out fuse?? Ordinarily you are not allowed to use this fuse to protect your installation, it is there purely to protect the service cable from overload even if it does appear to be "at the wrong end" if you see what i mean.. It is not your worry, it is "external" to "your" bit, it is all to do with the DNO. So long as you get a low enough Zs to protect "your" bit, that is, the installation protected by the RCBO, that is all you need to worry about.

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john..

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[Technician]

You should be in the Green one Zak.

 

[Technician]

BS 7430 is the COP for earthing.

 

[pep71]

no way will public lighting circuits be on a TT from a roadside feeder pillar fed from an underground service.

you need to look again

what cable is the dno service ? pvc concentric , lead sheath ?