How to certify a ring main with lighting circuit taken of a fused spur???

[DurhamSparky]

Record the Highest ZS you get from the circuit, IGNORE the 5amp FCU that only protects the cable and not used for disconnection times just over load on the cable

Refer to the 32A MCB that is in the CU for Disconnection times which i believe is 1.15 ohms unless its protected by a RCD then it can be up to 1667 ohms.

so your 1.27 ohms is WELL within limits for disconnection by a RCD

TNS system should be 16mm earth to the CU and the bonding in 10mm

 

[newbie111]

thanks steve, thats a great help to me. I did not realise the rcd increased the value up to 1667, thought that was just for TT system. Im learning a lot from you guy's so thanks again for all your help.

 

[newbie111]

one last question, lf using a wander lead connected to the earthing terminal firstly and then the line conductor(at the consumer unit) and measuring from those points to the line and cpc at the furthest lighting point and the corresponding terminals. will ,when added together with ze will equal zs?
zs=ze(R1+R2) would this be one way to calulate zs for the lighting circuit?

 

[malcolmsanford]

That is a novel way to get your R1+R2 using a wander lead, normally you only use the wander lead for R2. To save you trailing a lead around just link the LINE/CPC at the CU and then take your readings at the points to get R1+R2.

Yes quite right by getting your R1+R2 and then adding the Ze will give you your Zs. This is often the way it's done on initial verifications. For a PIR I have to admit I would do a live test espicially in a commercial/industrial enviroment. I do realise that by doing it live you can introduce parallel paths, but IMO that is not a bad thing really, as that will be the status of the installation if a fault occurs.

Now with RCD protection so prevelant and "soft" test being done to avoid tripping, you can produce very high results on a Zs. Though the flip side to this is becasue of the use of RCDs as additional protection the Zs reading is becoming less and less critical as 1667 is your important figure and if you can't achieve below this then you have serious problems.

 

[newbie111]

thanks malcom. For this installation which has a lighting circuit connected through a fused spur,should i do the figure of 8 for the ring and then test the furthest lighting point with line/cpc crossed connected at the furthest lighting point and that will give me R1+R2?
, thanks again .