couple of questions

[nicsin02]

1. having a debate at work about a ring circuit, spurring of a ring at the d/b surely makes the socket a radial circuit does it not meaning that a 2.5mm is protected by a 32a breaker, says in the regs book u can but i see it as a fault, what use guys think?

2.when measuring a Zdb at a D/B do you have to turn off the mains switch and test the live side as i have tired a few times recently leaving the board on and results were exactly the same.

3. was testing the Zs of a 110v transformer and the metal enclosure was not earthed, would you code 2 this?

4. when carrying out a Zs test on a metal clad socket and you are holding onto the earthed metal plate will this give you a shock if your body completed a circuit? just a wee debate at work, thanks guys

 

[kingeri]

1. A spur on a ring can come off any point, including the DB, the current drawn is limited by the load, that's why you can't spur off a spur.

2. Answered your own question.

3. Code 3 personally, but could justify a 2.

4. Try it.

 

[daiplayer]

4 - doubt it , it deosnt produce a touch voltage high enough . if it did it would trip the MCB when testing . ? trip lock testers keep this current under 30ma, ( 20ma more likely) so , at a stab in the dark. nope. but I could be ( and probably am ) WRONG ! LOL

 

[nicsin02]

1. A spur on a ring can come off any point, including the DB, the current drawn is limited by the load, that's why you can't spur off a spur.

2. Answered your own question.

3. Code 3 personally, but could justify a 2.

4. Try it.

I honestly think spurring from a MCB would be making a radial circuit, for example if the (r1+r2) of a radial was 0.35ohms then the spur would be 0.35ohms plus the resistance of the new spur, if i were to measure the (r1+r2) of the spur from the D/B i would imagine it wouldnt include the ring main resistance hence it would be its own circuit and the mcb over-rated, if one double socket was spurred from the d/b that would have a maximum draw of 26amps which is even borderline, but what happens if the client uses extentions or there is a fault on the cable etc.....

 

[fredrico]

I would not spur off a ring at the mcb. Look at app 15.

 

[somersetsparks]

I honestly think spurring from a MCB would be making a radial circuit, for example if the (r1+r2) of a radial was 0.35ohms then the spur would be 0.35ohms plus the resistance of the new spur, if i were to measure the (r1+r2) of the spur from the D/B i would imagine it wouldnt include the ring main resistance hence it would be its own circuit and the mcb over-rated, if one double socket was spurred from the d/b that would have a maximum draw of 26amps which is even borderline, but what happens if the client uses extentions or there is a fault on the cable etc.....

It is no different then spurring off of any other point on the ring IMO.
A double socket can only sustain about 19A for any length of time anyway, why would this make any difference, you could plug extensions into any spur anywhere on the ring with same 'risks'. The fact is not many appliances are actually 13A.

 

[yellowvanman]

I would not spur off a ring at the mcb. Look at app 15.

Appendix 1 is normative, and is thus a requirement.
All other appendices are informative, and are provided as guidance.

 

[ackbarthestar]

I would not spur off a ring at the mcb. Look at app 15.

1/ Perfectly acceptable to spur off of the rfc at the DB. As stated in App 15.

Short circuit is provided at the MCB and O/L is at the appliance/BS1362 fuse. All that has happened is the methods of protection have been separated out.

When you consider it the cable is unlikely to be subject to an overload as long as no more than one DSO is connected at the far end.

However, It does leave it wide open to conjecture as to why you can't use a radial circuit with a 32A MCB supplying one DSSO fed via a 2.5mm2 cable.....

2/ Zdb is assumed to be a DB fed via a sub-main cable within an installation. When the isolator to the sub main DB is off, you are then reducing the possibility of transferring a touch voltage, however small, to the final circuits downstream at the point of test .

3/ The casing to the Tx is most likely connected to the primary side. The secondary side will be isolated from the metal enclosure. see 411.8.4.1

4/ Unlikely that by touching the m/c socket while testing that you will receive a shock although you never know so better not to.

The worst case would be to place yourself in parallel with the circuit under test, the current flow through the person conducting the test will be considerably less that that flowing in the conductors

 

[Brightspark2]

but what happens if the client uses extentions.

Then the extension will have a 13A fuse in the plug...

If the cable developed a fault then 100s or 1000s of amps would flow and the OPD would operate. Think PFC...

The socket would be classed as a spur...


Sent from my Xperia S using next doors WIFI.

 

[nicsin02]

1/ Perfectly acceptable to spur off of the rfc at the DB. As stated in App 15.

Short circuit is provided at the MCB and O/L is at the appliance/BS1362 fuse. All that has happened is the methods of protection have been separated out.

When you consider it the cable is unlikely to be subject to an overload as long as no more than one DSO is connected at the far end.

However, It does leave it wide open to conjecture as to why you can't use a radial circuit with a 32A MCB supplying one DSSO fed via a 2.5mm2 cable.....

2/ Zdb is assumed to be a DB fed via a sub-main cable within an installation. When the isolator to the sub main DB is off, you are then reducing the possibility of transferring a touch voltage, however small, to the final circuits downstream at the point of test .

3/ The casing to the Tx is most likely connected to the primary side. The secondary side will be isolated from the metal enclosure. see 411.8.4.1

4/ Unlikely that by touching the m/c socket while testing that you will receive a shock although you never know so better not to.

The worst case would be to place yourself in parallel with the circuit under test, the current flow through the person conducting the test will be considerably less that that flowing in the conductors

In a purely theoretical situation what would happen if I spurred of mcb too a double socket, 1x extension with four outlets plugged into each side of the socket and all 8x outlets were getting used and running at a max of 13a each? For me it means that both cable and double socket is in danger of burning? I know this is purely theoretical but I have seen some crazy extension leads setups at offices etc..... Just seems like bad practice too me, I may be wrong or it's perhaps a grey area, wot u guys think, cheers

 

[ackbarthestar]

In a purely theoretical situation what would happen if I spurred of mcb too a double socket, 1x extension with four outlets plugged into each side of the socket and all 8x outlets were getting used and running at a max of 13a each? For me it means that both cable and double socket is in danger of burning? I know this is purely theoretical but I have seen some crazy extension leads setups at offices etc..... Just seems like bad practice too me, I may be wrong or it's perhaps a grey area, wot u guys think, cheers

Interesting....
Eight 3kW appliances run concurrently connected directly to a 32A rfc would load the circuit to 104A. Very unlikely unless someone was planning an attic greenhouse of exotic plants.

Office machinery such as PCs, printers and the like on two 4 way extension leads would use no more than a couple of amps. So probably not so theoretical as you might imagine.

My thinking is that the regs (BS7671) veers on the side of caution. So, IMO, there's not too much to worry about connecting a non-fused spur to the origin of the circuit.

 

[HandySparks]

In a purely theoretical situation what would happen if I spurred of mcb too a double socket, 1x extension with four outlets plugged into each side of the socket and all 8x outlets were getting used and running at a max of 13a each? For me it means that both cable and double socket is in danger of burning? I know this is purely theoretical but I have seen some crazy extension leads setups at offices etc..... Just seems like bad practice too me, I may be wrong or it's perhaps a grey area, wot u guys think, cheers

If all eight sockets were supplying a 13A load, the 13A fuses in the plugs connected to the fixed double socket would blow. That's what allows the use of a double socket on a 2.5mm² spur, you can't overload it.

(Yes, I know that some manufacturers state a lower limit than 26A for the complete fitting.)