Outside lighting circuit Question

[sambotc]

Just changed an outside light and was just questioning the max Zs allowed. It has been spurred off the ring main, switched fused spur with a 5A fuse in (existing). Final ring circuit is protected by a 32A MCB and RCD protection.

When carrying out Zs at the light fitting (cable runs under patio and up wall in conduit) I had a high Zs at 18 ohms. Tested using 'loop' on tester. When I checked using 'line' the reading was fine, so I presume it is a poor connection on the earth somewhere unless I am missing something?

I am going back tomorrow as I ran out of time today to check out why the reading was so high, as it is RCD protected theoretically it is acceptable, right? (1667) but I would rather find out the issue rather than relying on the RCD.

So when it comes to max Zs, do I go by a 6A MCB, or a 5A BS1361?

Appreciate any advice

Thanks in advance

 

[telectrix]

might be an idea to measure R1+R2, or R2. (dead test). as for the OCPD, you go by the MCB in the CU. in this case 32A.

 

[brman]

Just changed an outside light and was just questioning the max Zs allowed. It has been spurred off the ring main, switched fused spur with a 5A fuse in (existing). Final ring circuit is protected by a 32A MCB and RCD protection.

When carrying out Zs at the light fitting (cable runs under patio and up wall in conduit) I had a high Zs at 18 ohms. Tested using 'loop' on tester. When I checked using 'line' the reading was fine, so I presume it is a poor connection on the earth somewhere unless I am missing something?

I am going back tomorrow as I ran out of time today to check out why the reading was so high, as it is RCD protected theoretically it is acceptable, right? (1667) but I would rather find out the issue rather than relying on the RCD.

So when it comes to max Zs, do I go by a 6A MCB, or a 5A BS1361?

Appreciate any advice

Thanks in advance

Depends a bit on what earthing system and what your loop impedence is at the origin.
If L-E is ok at the orgin but high at the light (compared to L-N) then yes, I'd say a dodgy connection. So regardless or RCD or not there is a fault that needs fixing imho.

For Max Zs, if the MCB gives the required times to the FCU and the MCB or the FCU give the required times for the extension then I'd be happy. I'd be slightly less happy but still accept it if the RCD gives the required times, but only if there isn't a fault. I am aware this might be a slightly controversial view

 

[Octopus]

There was a long thread on this very subject a couple of days ago - whether to use the MCB or the FCU fuse - I'd say opinions were divided!

 

[Barry Rathbone]

You can change your fuse to a 3A and your in @23.2 ohm for a fixed appliance.Unless you can't of course due to high wattage?!
Would be better to investiate as you say though.

 

[johnboy6083]

If no RCD, then use the max Zs of the fuse, not the MCB. 9 times out of 10, the MCB will operate first, unless there is a substantially long cable from the FCU to the light ect.

 

[telectrix]

for max. Zs, you use the MCB at the origin of the circuit, not some fuse half way round the RFC. if you look atr the shedule of test results on a MWC, you will see that it asks for the details of the modified circuit. the Zs you are measuring is for the circuit, not just the spur from it.

 

[Octopus]

So if the lighting spur is off DB2 do you measure the Zs from the MCB on DB1? Tin hat on

 

[pencilpusher]

There was a long thread on this very subject a couple of days ago - whether to use the MCB or the FCU fuse - I'd say opinions were divided!

and here it is!

http://www.electriciansforums.net/e...ions/59694-voltage-drop-zs-some-thoughts.html

 

[sambotc]

wow, thanks for the replies so far, just to recap.

Sorry typo in original post, I mean't 32A MCB not 6A... doh

I did test R1+R2 at the beginning come to think of it and it was fine but I tested at the switched fused spur with the light end joined witha connector block, which would lead me to believe the fault must be at the switched fused spur, as I checked Zs at the socket which had been spurred off and all was fine. 0.50

The cable in question is 1.5mm and is approx 10 metres long. It was run in flexi type conduit (probably pond pipe or similar) and the end wasn't sealed, so initially I thought there may have been some damage to the CPC.

Seems to be some conflicting opinions on the Zs bs1361/MCB. Surely either would suffice logically? I will have another look in the morning and see if it is the spur causing the issues.

I did have a quick visual check and all looked ok, and I tried to test Zs from the load side but wasn't sure if the leads were making a good connection due to furniture being in the way. I'd rather not put a 3A fuse in to make a fault pass, would rather fix the fault if possible. Replacing the cable is not an option either.

 

[telectrix]

So if the lighting spur is off DB2 do you measure the Zs from the MCB on DB1? Tin hat on

i say that you need the value for that circuit's OCPD. therefore you would measure the Zs at DB2 and deduct this from the measured Zs at the accessory.

what i mean is, that the total Zs is made up of 3 parts, Ze + Z (cable from DB1 - DB2) + Z (circuit cable from DB2 to accessory)

 

[brman]

Seems to be some conflicting opinions on the Zs bs1361/MCB. Surely either would suffice logically?
​

well that is my view but I've yet to win an arguement with teletrix so I'm still waiting for someone to point out why I am wrong...........
(but the "what you have to put on the cert" arguement doesn't quite do it for me. I want to know what is wrong with it from a design point of view)

 

[Barry Rathbone]

MCB 32A Type B Max Zs= 1.44ohm....

Good luck getting it that low...

 

[brman]

i say that you need the value for that circuit's OCPD. therefore you would measure the Zs at DB2 and deduct this from the measured Zs at the accessory.

what i mean is, that the total Zs is made up of 3 parts, Ze + Z (cable from DB1 - DB2) + Z (circuit cable from DB2 to accessory)

But it is still total Zs that you are checking right? (i.e. loop impedence right back to the substation)
But in that example you check it against the OCPD at DB2 because that is what is protecting the circuit. I still don't see how this differs from the FCU example.

 

[soulman]

hi, if you got a reading of 18ohms this obviously it not correct (unless tt) and unproffessional to leave it because there is an rcd device. It is to my understanding if you are adding to a circuit & including a new protective device such as 1362 fuse. Then your zs value should be from the fuse. Why would the zs values be in the regs for this type of fuse. Minor works cert is asking for the information of the circuit you are extending to ensure the circuit complies before you extend it. This probably wont matter anyway if the circuit is covered by an rcd.

 

[johnboy6083]

for max. Zs, you use the MCB at the origin of the circuit, not some fuse half way round the RFC. if you look atr the shedule of test results on a MWC, you will see that it asks for the details of the modified circuit. the Zs you are measuring is for the circuit, not just the spur from it.

I disgree tel I would class the FCU fuse as a new circuit. So another way of lookinmg at it would be to imagine the circuit that the FCU is fed from as a submain. If the VD is within spec, disconnection times are met, and the Iz of the cable is ok, then all is good.

 

[telectrix]

details of the modified circuit is for details of the circuit after it has been modified, not as it was before

 

[sambotc]

MCB 32A Type B Max Zs= 1.44ohm....

Good luck getting it that low...

Zs at the ring final is 0.50. cable is 1.5mm T&E and say 10mtrs long. Table I1 in GOSG suggests a resistance of R1 + R2 = 30.20m ohms/mtr

30.20 x 10 = 0.302 ohms? + 0.50 = 0.80 ohms

Is that correct?

I'm hoping/thinking its the switched fused spur causing the issues.

Surely disconnection times are there to ensure safe operation of the over current device under fault conditions, so providing the circuit is made safe, regardless of which device does this, the objective has been achieved?

yet another grey area in the world of electricity!

 

[sambotc]

Just to add its TNCS Ze of 0.10, so yeah definitely a fault somewhere that I will try my hardest to find.

 

[Barry Rathbone]

Your first line of protection is your 1361 fuse, NOT the 32A.... your are a couple of ohms out from a .4 disconnection time on a 3A fuse.
So check connections/cable and it wiil be fine...