Ring vs Radial circuit, what would you do?

[JohnW2]

We consider sockets on a 32 amp supply as having a design current of 26 amp, that is 20 amp centre of ring and 12 amp even spread ...

Although it's probably a fairly reasonable 'nearly worst case' scenario, I think that "We" relates to some IET person you once heard speaking?

.... so for volt drop 26 amps is used, allowing up to 106 meters of 2.5 mm² cable to be used. .... But split that into two with 20 amp overload, and we then work on the full 20 amps, so again with 2.5 mm² cable, we can have 32 meters of cable.

That's the approach the OSG takes. However, as I recently wrote, those 'maximum circuit lengths' result not only from consideration of voltage drop but to the suggested 'guidance' figures for maximum VD. I don't think many people are particularly concerned about these VD considerations and, in any event, the calculation of 'maximum theoretical VD' relate to a situation which will almost never arise - with the circuit loaded right up to it's capacity (e.g. 32A for a ring) and, even then, a VD in excess of the 'guidance' figure could only ever happen at the midpoint of the ring.

As I recently wrote, if one considers Zs (much more important) rather than VD, then the maximum circuit lengths can be quite a lot greater than you or the OSG suggest

A circuit for which calculations indicate the the VD at the worst point (centre of ring or end of radial) could theoretically exceed the 'maximum' guidance figure will (provided Zs etc.is satisfactory) can be fully compliant with BS 7671, provided only that the voltage drop does not "impair the safe functioning of connected equipment)

This would show an increase of 0.59 Ω on the loop impedance reading. .... This would show an increase of 0.79 Ω on the loop impedance reading. ....
with an increase of 0.97 Ω on the loop impedance reading.

"Increases" relative to what?

 

[JohnW2]

I’ve been called in to “fault find” after such work - generally by the wife who tells me the husband did it

Was it perhaps also the wife who had tested the installation and determined that there was a fault that could be addressed by splitting a ring?

 

[ericmark]

That's the approach the OSG takes. However

I also gave the "however", and did say how volt drop may not matter. But we on the forum simply don't know. For all we know, a 100-meter ring final may have been split with 10 meters on one leg, and 90 meters on the other.

I remember inspecting and testing a 4 mm² radial, on a 30 amp fuse. Only to find in the middle, someone had split the radial and used 2.5 mm² cable to add another 6 sockets in the room, clearly the electrician had thought he was working on a ring final, and yes electrician as commercial premises with a maintenance electrician always on site, so not DIY work. He clearly also did not test, again that was clear, as when I arrived I found no test equipment other than a multimeter, and an old out of date PAT tester.

So I swapped to a 20 amp fuse, and flagged it up as to do work. But before I got around to doing it, I left the firm, but that means someone will have to work out why a 4 mm² radial is on a 20 amp fuse.

These things happen, and if that was posted on a forum, it is very likely the wrong advice would be given, as not what one expects to find.

 

[Murdochcat]

I find that situation a bit stressful...maybe thats just me.

I just thank the customer for their honesty and point out that by doing that they normally make the job easier and cheaper to fix

 

[AndyPRK]

So does your CU contain 1 RCD, 2 RCD's or all RCBO's.

RCDs and RCBOs can be identified as they have a Test button

 

[EFLImpudence]

RCBOs are RCDs.

 

[JohnW2]

RCBOs are RCDs.

They are, but I'm not sure what we are meant to call those devices that we called "RCDs" long before anyone had even dreamed of RCBOs.

You would probably say "RCCB" - but does an RCBO not qualify as one of those as well?

 

[EFLImpudence]

They are, but I'm not sure what we are meant to call those devices that we called "RCDs" long before anyone had even dreamed of RCBOs.

You would probably say "RCCB" - but does an RCBO not qualify as one of those as well?

If you really want me to explain, I will.


However, it would be better if replies were written so that DIYers can understand which device is meant.

 

[JohnW2]

I also gave the "however", and did say how volt drop may not matter.

You did, and my personal view is that, in a domestic environment, it is virtually never going to matter at all. One of the things you wrote was,,

.....Of course, one can ignore the recommended 5% volt drop limit, and to be frank only items using a refrigeration unit with does not have an inverter drive are likely to have a problem with volt drop. This is why we are told not to run freezers etc, using an extension lead, and although our supply is 230 volts - 6% + 10% so 216.2 volt to 253 volt, in real terms we likely see around 244 volts ...

Indeed, but as you imply, I really do doubt that even fridges/freezers (without inverters) are often going to experience problems due to VD, given that most people have supply voltages of 240 V or more, which is about 15% more than the ~205 V (216.2 V - 5%) at which the appliance is presumably expected to work satisfactorily.

 

[JohnW2]

<a quote of my post>[/quote

Have you something to say - perhaps an answer to my question?

 

[EFLImpudence]

Look again - some magic has happened.

 

[JohnW2]

If you really want me to explain, I will.

If you're saying that you think that using "RCCB" would exclude devices that have over-current, as well as residual current,functionality, as I implied (actually 'asked') is that actually correct?

However, it would be better if replies were written so that DIYers can understand which device is meant.

I think the issue you raised essentially only exists for non-DIYers (or those few DIYers with more-than-usual technical knowledge). I would suggest that the great majority of DIYers do understand what devices are being referred to as "RCDs" and "RCBOs", it only being 'a problem' in the eyes of those who are trying to be 'terminologically clever' (and, ironically, thereby probably confusing some DIYers)

 

[JohnW2]

Look again - some magic has happened.

Who did that, and how?

 

[Jurassicspark]

Currently:
Downstairs: 16A radial in 2.5T+E (was a ring when rewired, but presume tested bad so someone swapped MCB for a 16A, I found the other end still live under the floor with a single wrap of insulation tape....)

What if it was not a ring, originally wired or rewired, and that cable is just a remains of which was a spur or anything else?

Kitchen + Garage: Joined into the same MCB as downstairs sockets (this flagged on EICR) also 2.5 T+E

What was the MCB size for this circuit? And what exactly was flagged on the EICR, which would raise any sort of concern?

We already have a dedicated circuit for one electric fire (other room planned to be gas), so likely only appliances on the downstairs circuit will be low power TV/Game console/lamps/etc. I understand that a short enough radial can possibly be upped to 20A too.

Horses for courses.

 

[EFLImpudence]

If you're saying that you think that using "RCCB" would exclude devices that have over-current, as well as residual current,functionality, as I implied (actually 'asked') is that actually correct?

Yes.

In the beginning we only had one device that was an RCD so we just called it an RCD.
Then RCBOs arrived so we had to distinguish between the new arrival and the previous sole device so that is now called an RCCB.

It might not be the definitive description but that is the situation and you know all that as well as I do.

I think the issue you raised essentially only exists for non-DIYers (or those few DIYers with more-than-usual technical knowledge). I would suggest that the great majority of DIYers do understand what devices are being referred to as "RCDs" and "RCBOs",

Ok. I don't know how many people you have asked.

it only being 'a problem' in the eyes of those who are trying to be 'terminologically clever' (and, ironically, thereby probably confusing some DIYers)

No - 'terminologically correct' or 'accurate' and not confusing - nothing 'clever' about it.
It's strange how you are so (sloppy and) uncaring about accuracy in the things about which you don't care - i.e. things I correct - but adamant about things on which you "comment".