Crossover in ring circuit

[KeenPensioner]

HI, as my profile says I'm not a sparky and know my limitations. I'm trying to learn (for the sake of it only as I'll never do it) what tests for "Ring Final Circuits" are. I understand how the Tests 1, 2 and 3 are done but in one of the videos I watched the presenter (John Ward) mentions a "crossover" see image and says that this "could lead the current being shared in an undesirable fashion. I don't understand that.

 

[The Ghost]

That single piece of cable in the ring cannot carry the full current on its own as much as the "doubled up cable" effect of the ring.

 

[Wilko]

The ring is a bit of an historic artefact, but a good one in my view. With the middle cross over it is no longer possible to perform a single test end to end. It doesn't improve the current capacity but it does make it much harder to test. My quick thought is it won't ruin the current handling capacity of the ring, but it's not the construction method shown in Appendix 14 of BS7671.

 

[The Ghost]

Actually thinking about it I retract my previous statement. Pretty stupid really.

 

[Taylortwocities]

It's not done strictly in accordance with the good book, but there are worse arrangements!

The major issue is that, when you do testing on a ring final, you crodd connect the conductors in a sort of figure of 8 arrangement. Then you test at each socket and check the R1+R2 resistance. In a properly installed ring final, the R1+R2 at each socket should be more or less identical.

The circuit would fail this test because of the the 'lollypop' arrangement in your post.

Learn more about this test HERE

Or, if you love John Ward

 

[Deleted member 26818]

It’s really too complicated to explain.
Also your diagram is not ideal.
Having a bridge reduces the resistance, so that more current can flow along one leg of the ring.
Current that should be in one leg gets diverted to the other leg which could result in the other leg being overloaded.

 

[HandySparks]

Current that should be in one leg gets diverted to the other leg which could result in the other leg being overloaded.

Although I think you'd be hard pushed to find a case where an overload has actually occurred due to a link in a ring.

 

[Deleted member 9648]

TBH I cant really recall any damage to cables on any of the hundreds of incorrectly wired rings I've come across over the years. In fact in the domestic sector it's pretty rare to find a correctly wired ring unless the house is fairly recent. The one I'm rewiring at the moment has spurs off spurs off spurs in the kitchen (now ripped out), no sign of thermal damage at all.

 

[kingeri]

I think in reality it's highly unlikely that an incorrectly wired ring in a domestic setting would ever see high enough loading of sufficient duration to actually cause any cable damage. Seen a fair few 2.5mm^2 and even a few 1.5mm^2 radials on 30A/32A OCPDs and don't recall seeing any evidence of overload damage. Obviously, it's not good though!

 

[Deleted member 9648]

Yayyy! I got a dumb!

 

[Deleted member 26818]

I get them every so often.

 

[kingeri]

Yayyy! I got a dumb!

I can't see why though!

 

[Taylortwocities]

I wouldn’t worry about it. Check out the profile of who gave you the rating!...
Only one post since 2012, and two dumb ratings
I’ve reset the balance for you

 

[Pete999]

I wouldn’t worry about it. Check out the profile of who gave you the rating!...
Only one post since 2012, and two dumb ratings
I’ve reset the balance for you

It's amazing in some respects, you answer a post get, say 3 likes and a couple of agrees and 1 Dumb, that's like saying all the likes and agrees are dumb as well, yes I always check the profile, some are genuine,most are as you say TTC

 

[Upton Sparks]

Yayyy! I got a dumb!

Bit Harsh I thought as well.

 

[Upton Sparks]

Its an equation at the end of the Day,

The whole idea of the ring is at any socket outlet the resistance should be the same back to the Fused board.
Therefore the current will travel both way back to the fuse board at an equal 50% each way down each leg of the ring. by adding a connection between the sockets and creating a figure 8 the resistance will now be imbalanced. therefore the current will now be unevenly splite. ie 40-60% 30 -70%, it can even get to 10- 90%, depending on the resistance of the Legs, it would be a very difficult job to access this and require a level of mathematics beyond just looking at tables.

and the reason we don't see any damage is most domestic house don't use a lot of current of their ring mains.
But should they try to use the 32 amps on offer then they will most certainly start to get issue,

 

[Rpa07]

I can't see why though!

I’m sure most ‘dumb’ ratings are people scrolling through on iPhone - too easy to do. Anyway - don’t let it get you down!

 

[HandySparks]

The whole idea of the ring is at any socket outlet the resistance should be the same back to the Fused board.
Therefore the current will travel both way back to the fuse board at an equal 50% each way down each leg of the ring.

What???

The resistance of a section of cable between the origin of the ring and the load will be proportional to the length of that cable. So a socket near the origin will pull more current along the 'short' route than the 'long' route, in inverse proportion to the resistance.

 

[Deleted member 26818]

The R1 + R2 should be the same at each socket (except spurs) when testing, because the conductors are joined together into a big circle.
Current sharing in normal use, will be as Handyspark says proportional to the resistance of each leg.
The longer the leg, the greater the resistance, the lower the current.

 

[Upton Sparks]

Ok Boys back to the testing books for you I'm afraid.
After doing your ring continuity, I hope you then do your Figure 8, You then test each socket to obtain your R1+ R2, your expecting the resistance reading R1 + R2 of the Ring to be the same at every point!.(Give or take 0.05)
therefore the current will flow equally back to the board regardless of where you are on the Ring,( its the same resistance either way its a ring) thats the science behind and why you do the Figure 8 testing and check every socket..
If you have high reading its normal a spur, or you've connected it up wrong, (normally only happens on Single..... unless your a really idot). Or the circuit is not a ring.