Rule of thumb v calculations

A BS 3036 fuse will actually allow 2 * In for up to 4 hours - we used to call it coarse excess-current protection :D.

Maybe just as well, if failed easier the asbestos flash pad might be disturbed more often :LOL:

I don't agree with the dingbat that we should ban ring finals, they do have plenty of uses, but I do agree that they are sometimes used where they are not the best design... but I've also seen poorly designed radial circuits as well (granted they tend to be lighting rather than sockets though)
 
Sponsored Links
And this discussion is yet another great reason for abandoning rings altogether.

And also a great reason to provide more training to people who specify the electrical installation to enable these people to be able to design the installation instead of creating a plan for the installation that contravenes the fewest guidelines.

If it was a simple and reasonable circuit design, none of that confusion would arise.

If designers of electrical installations had better understanding of electrical theory then there would be less confusion.

Recently a qualified electrian member of a part P self certifying organisation could not explain how an RCD could protect a person using a double insulated tool in the garden. He as much as said if there is no earth wire in the cable then the RCD cannot detect earth faults. No doubt ( or maybe very little doubt ) his installations complied to the rules and guidelines.

Bernard
 
I don't agree with the dingbat that we should ban ring finals, they do have plenty of uses, but I do agree that they are sometimes used where they are not the best design.

Any circuit design that is safe and provides a well-engineered solution should be acceptable, but the original concept for ring final circuits for domestic socket-outlets was fundamentally flawed. The average electrician should not be encouraged to use a design he does not understand and given that many struggle with Ohm's Law... :rolleyes:
 
Sponsored Links
The average electrician should not be encouraged to use a design he does not understand and given that many struggle with Ohm's Law... :rolleyes:

If that was a BS 7671 requirement then there would be a serious shortage of "electricians" ,

Ah no....that doesn't apply...... I now see you said ""should not be encouraged to use" I thought I had read " should not be allowed to use "


( that was with sarcasm turned ON )
 
A BS 3036 fuse will actually allow 2 * In for up to 4 hours - we used to call it coarse excess-current protection :D.

Mind you I can't remember climbing over too many bodies killed by ring mains - can you?

I bet you remember "Close excess-current protection " too :D.
 
A BS 3036 fuse will actually allow 2 * In for up to 4 hours - we used to call it coarse excess-current protection :D.

Mind you I can't remember climbing over too many bodies killed by ring mains - can you?

I bet you remember "Close excess-current protection " too :D.

He doesn't remember it, it was 1.5x In for 4 hours.

Oh yes he does - you are quoting a definition given in the metric version of the 14 Edition (it all went wrong when we went metric :D).

It is not incompatible with the statement I made as it is saying will not operate at 1.5 times within 4 hours - whilst I was using a definition that says that it will operate at 2 times in that time.

Now BS 3036 says:
Fusing factor is defined as Minimum Fusing Current / Current Rating

For BS 3036 fuses it must not exceed 2 (clause 7 of BS 3036).

Minimum fusing current is defined as the minimum current that will melt the fuse.

The test is describe in clause 19 of BS 3036 – basically the fuse should operate under defined conditions in a time not exceeding that give in Table 5 of BS 3036 (this time is 1.25 hours for a 30A device).

The test is actually conducted twice using two different currents – one 90 % of the other.

The fuse must melt at the higher current within the specified time. It must not melt at the lower current within the same time. The minimum fusing current is the arithmetic mean of these two currents.
 
Simple Q, and it would benefit everyone to try and answer it as seriously and honestly as possible - not here, but just to themselves.

If ring finals did not exist, and someone came up with the idea that if we connected sockets in a ring then as long as the cable was rated at ≥ 20A we could use a 32A breaker, do you think that the IET would go for it, and professional electricians would say "Fantastic idea - I'm going to start installing them as often as I can"?
 
I am a "Ringer" I install rings.
There are pros and cons with them.
They have stood the test of time.
They can be part of a well engineered system.

Having said all that - Mr Sheds old son you have a very valid point here , if a ring final was not already a concept then would it become one?" Not very likely " I'd say is my honest opinion.

Also Happy new year too - everybody
 
If ring finals did not exist, and someone came up with the idea that if we connected sockets in a ring then as long as the cable was rated at ≥ 20A we could use a 32A breaker, do you think that the IET would go for it, and professional electricians would say "Fantastic idea - I'm going to start installing them as often as I can"?

The question is simple. To arrive at a fit all situations answer is not simple.

The single fault tolerance that does not create a risk of fire from a failed terminal is important. The dual route for the CPC to reduce the risk of a lost CPC is also important.

The process of arriving at an answer is complicated by the factor of time. Most cables will safely carry twice their rated current for a period much longer than the reaction and disconnect by melt time of a wire fuse of the same rating as the cable. If they didn't then the cable would melt before the fuse melted. The simple rating of 5 amp fuse wire covers a non linear time against current graph. As does the reaction time of an MCB with its dual method of operation. So for simplicity it is called a 6 Amp MCB

Ask yourself what happens in a 20 Amp rated cable that is overloaded between the start of the overload and the time at which the thermal trip in the MCB operates. The cable can be carrying 28 amps and for that short duration of time is according the the BS 7671 standards safe otherwise the BS 7671 would require that the MCB was reacted more rapidly.


that is not a reason to design at 1.4 of the cable's rating because there is also the factor of the thermal inertia of the cable to be considered in calculating how long a cable can carry 1.4 times its rated current.
 
A lot of the fault scenarios mentioned would be aleviated by banning T+E cable.

It is only very recently that a manufacturer has introduced a stuffing gland with an T+E shaped insert. A nice innovation but totally incompatible with standard plastic accessories.

So for most of the past 60 years a cable has been in standard use with no method of securing the cable sheath. How was it ever approved in the first place? Why does it have reduced size earth? How many fires/accidents have been caused by these unsecured cables?

If the only acceptable cables were properly secured armoured or metallic sheathed we could also do away with totally nonsensicle safe zones. Also compulsory use of rcd's (with a laughable 8% failure rate) on lighting circuits could be abandoned
Not sure on the securing as I have used the metal strips for years with the slit in them you screwed or nailed them in and placed cable on them and secured similar to today's tie rap.

As to design the idea of putting two cables inside a metal sheave is as old as the hills it was when the outer lead covering was replaced with plastic that the rot set in.

The later adding of a centre core to be used as earth did improve the design slightly but the only way to return to the original is to replace the plastic outer with metal again.

This has been done and the Ali-tube range of cables designed to go inside stud walls Guardian, Earthshield, Flexishield, and Afumex are bring us back to the days when cable had lead covering although not sure if eating aluminium will kill rats in same way as lead maybe arsenic impregnation like the railways used is also required? Rats don't tend to eat lead cables.

The problem is supply and price. There are jobs where, until amendment 1 comes out which is it seems going to allow adding to a non RCD protected circuit, to be able to place the RCD at the socket end wiring in Ali-tube would make sense. However go to a DIY shed and ask for Ali-tube and one is met with a blank expression. So one has to buy it by the role. And it is expensive and most electricians don't want to buy roles of BS 8436 Ali-tube to use with the odd extension to a ring main.

The design with both outer earthed and a bare earth wire within does raise more questions as only the inner core is copper so we should not really include the outer tube with our earth loop calculations. However since the two are in contact with each other it is impossible to take separate readings for copper and aluminium so although the installation may pass when installed it is far more likely than twin and earth to deteriorate over time and then fail the ELI test.

We look in the 17th Edition and there is a page devoted to twin and earth and how it is to be rated when used in insulating walls (Table 4D5) but there is no table to give us the ratings for Ali-tube which was to have replaced twin and earth.

The temperature ratings are another matter and although Ali-tube is rated at 90 deg C where more twin and earth is rated at 70 deg C there is nothing to tell us if the wood beam we are attaching it to is rated at 90 deg C. The same applies to mineral insulated which I grant you is great for hiding in the mortar in a stone cottage so no cables are seen but can the oak beams take the temperature? It does of course give great ELI readings with the copper outer but as far as volt drop goes a ring main with 1.5mm cables it must be much reduced and can't see 100 sq meters being covered with 1.5mm live conductors in the cable!

I do see how round cables look far better on tray work and never look twisted and I have seen the Ali-tube being used in commercial buildings but in domestic I would think rare and where it is used in the main it's used as a cheap replacement for mineral cable in listed buildings.

But the question is really about if we should now be doing more calculations with the use of the RCD meaning the ELI can be very high around 200 ohms and circuit still pass it is now the line / neutral impedance which limits the length of a ring main due to volt drop. And the length of cable used in ring mains has increased slightly due to this to for a B32 MCB 106 meters. With this in mind I would have expected to see the prospective short circuit current being entered on the paperwork as this is a direct relationship to the volt drop.

However the IET forms do not require this to be recorded and even if they did many of the meters do not measure line / neutral but just measure line / earth for both readings. Yes I know top of range Robin does measure line / neutral but there are many which don't. If they did then flicking between the two readings would not give I x R = 230 as the "I" would be two x 2.5mm and "R" at 2.5mm and a 1.5mm. Yet so many meters multiply them together and one gets 230 with twin and earth. Or even worse 240 where conversion has not been recalibrated.

I play safe and use the wandering leads with an unknown meter and until tested I do not assume in meter switching where moulded leads are used.
 

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.


Are you a trade or supplier? You can create your listing free at DIYnot Local

 
Sponsored Links
Back
Top