reusing a disused shower circuit

[Goldberg]

2. Why do you believe that a 40A CPD is ok for my proposed radial with a 5A loading?

because the cable is sized correctly for a 40A CPD and the load is fused down at the connection point with an FCU.

And what happens if an extra 35A is drawn - where will that safely go?

3. Can you think of any risk that arises when using a 30A breaker/cartridge on jpbunce's radial final, that either doesn't arise, or is to any extent mitigated, when using a 10A CPD?

no I can't ( as I've already said at least twice in answer to this question you KEEP asking ), but likewise I see no risk arrising from leaving the 30A in place and connecting the equipment with FCU's as is the OP's intention..

I apologise if you've had to repeat your answer, but that's the first time I've noticed you saying that you can't think of any risk.

I really thought you would, but there's still a chance that someone else will.

 

[Goldberg]

Not forgetting the cable is supplying fcu's.
Why feed 1 or 2, 13amp rated fcu's with a 10 amp mcb.

Did you think that the design load was 26A?

The 30 amp is already in place, he may in the future want to increase the heater size or fit a larger towel rail, he will then have the facility to increase up to 3 kw each if so desired.

If he keeps the old breaker then he won't lose that facility. In the meantime, do you think that the total design load for his two fixed appliances is 26A?

Do you think that it's perfectly fine for 30A to be drawn when the FCUs loads are fused down to a total of 8A? Where are the other 22A being safely used?

The fuses in the FCU will limit the fault current the same as a plugfuse on a 30 amp ring.

Each fuse would limit the fault current if the fault was on the load side of its respective FCU. I don't any way for either fuse to limit any other fault current.

What would be good discrimination in that example?

Where in the event of a fault, only the faulty appliance fuses and not tripping the circuit causing loss of other appliances also.

Do you envisage that a 3A of a 5A fuse would fail to break before a Type B 10A MCB?

If not, then why would such a breaker not fit into your model of good discrimination?

 

[ColJack]

how can I have misunderstood what you wrote?

you ask where the extra 22A goes to .. it doesn't get drawn in the first place..

how would I prevent 30A from being drawn by a circuit with 2 loads, one on a 3A FCU and one on a 5A FCU?
well when the load on the 3A FCU draws more than 3A under fault, the 3A fuse blows and cuts it off..
same with the 5A load on the 5A FCU..

And what happens if an extra 35A is drawn - where will that safely go?

it wouldn't go anywhere....
if in the unlikely event that a fault developed that drew 35A that didn't involve the load, then it would do nothing... the cable is spec'd to take that current with no problem..
same as a fault that drew 80A occuring in the CU on the 25mm supply cables.. the cables are spec'd for that current..

 

[Goldberg]

no I can't ( as I've already said at least twice in answer to this question you KEEP asking )

Looking back, I still think you haven't answered this question:

1. Imagine a radial circuit with a design load of 5A - let's make it easy and say the conditions make it acceptable to use a type B 6A MCB and 1.0mm² cable.

If I then chose to use 6mm² cable, for my own secret personal private secret private reason, would you then change the CPD to a 40A MCB?

If not, then why not?

I apologise if you have answered it - please could you point me towards your answer so that I can read it?

 

[Goldberg]

how can I have misunderstood what you wrote?

I don't know. I just thought that it was possible.

you ask where the extra 22A goes to .. it doesn't get drawn in the first place.

Are you saying that there are no circumstances whatsoever where a current greater than 8A can be drawn on a circuit with a design load of 8A and a 30A CPD?

how would I prevent 30A from being drawn by a circuit with 2 loads, one on a 3A FCU and one on a 5A FCU?
well when the load on the 3A FCU draws more than 3A under fault, the 3A fuse blows and cuts it off.
same with the 5A load on the 5A FCU.

That's certainly true of a fault on the load side.

And what happens if an extra 35A is drawn - where will that safely go?

it wouldn't go anywhere.
if in the unlikely event that a fault developed that drew 35A that didn't involve the load, then it would do nothing... the cable is spec'd to take that current with no problem.

So, in what circumstances could such a cable draw the extra 35A?

same as a fault that drew 80A occuring in the CU on the 25mm supply cables.. the cables are spec'd for that current..

Interesting.

Here's a photo of a cable that was spec'd for a current greater than the rating of its upstream cut-out, but it appears not to have coped very well:

View media item 13137
Do you have any explanation for that?

 

[ricicle]

The OPD is there to protect the cable only, not the load.

 

[ColJack]

and another misquote by you...

I apologise if you've had to repeat your answer, but that's the first time I've noticed you saying that you can't think of any risk.

really? so you don't read the replies then?

no there is nothing inherantly dangerous with putting a smaller breaker on an existing circuit...

no, nothing wrong with changing a breaker / fuse for a smaller one, but it is not a necessity.. there is no "MUST" be changed about it..
"CAN be changed" yes, but not absolutely MUST..

were both in reply to the question..

3. Can you think of any risk that arises when using a 30A breaker/cartridge on jpbunce's radial final, that either doesn't arise, or is to any extent mitigated, when using a 10A CPD?

and as the misquote you made was directly under the above question as a quote in the reply I can only surmise you are trying to bend the answers to suit yourself..

 

[Goldberg]

and another misquote by you...

I apologise if you've had to repeat your answer, but that's the first time I've noticed you saying that you can't think of any risk.

I apologise if I changed the meaning of what you posted - that wasn't my intention. I was referring to this reply of yours:

3. Can you think of any risk that arises when using a 30A breaker/cartridge on jpbunce's radial final, that either doesn't arise, or is to any extent mitigated, when using a 10A CPD?

no I can't ( as I've already said at least twice in answer to this question you KEEP asking )...

If you weren't replying "no" to the question "Can you think of any risk", then please could you tell me what you were answering "no" to?

I apologise if you've had to repeat your answer, but that's the first time I've noticed you saying that you can't think of any risk.

really? so you don't read the replies then?

Of course I read the replies, but it's possible that I've read the answer and then forgotten, and that I now can't locate it. I'm not a robot, and I was only asking you to point it out to me.

no there is nothing inherantly dangerous with putting a smaller breaker on an existing circuit...

I didn't ask if there was anything inherently dangerous in doing that.

no, nothing wrong with changing a breaker / fuse for a smaller one, but it is not a necessity.. there is no "MUST" be changed about it..
"CAN be changed" yes, but not absolutely MUST..

I didn't ask if there was anything wrong with doing that.

...as the misquote you made was directly under the above question as a quote in the reply I can only surmise you are trying to bend the answers to suit yourself..

I wasn't trying to bend anything, and I still think you haven't answered this question:

1. Imagine a radial circuit with a design load of 5A - let's make it easy and say the conditions make it acceptable to use a type B 6A MCB and 1.0mm² cable.

If I then chose to use 6mm² cable, for my own secret personal private secret private reason, would you then change the CPD to a 40A MCB?

If not, then why not?

 

[333rocky333]

View media item 13137
Do you have any explanation for that?

More likely a loose connection , it is rare to see meter tails burnt up like that due to overload.
The outgoing tail from the meter carries the same current and looks in no way damaged.

 

[ColJack]

without knowing the circumstances of that photograph it's impossible to tell for sure..
at a glance, that cable looks to be 16mm, not 25mm so if the fuse in the cutout is a 100A then it's too small..
failing that, if it IS 25mm then there may have been a loose connection inside the switchfuse wich would cause heating of the joint without an overload.... this would travel back up the cable..

as you say.. a 6mm cable is spec'd to take 47A ( clipped direct ), it is protected by a 30A fuse.. it feeds a combined 8A load..
if there is a fault load side then one of the fuses in the FCU would go..

if there was a fault on the supply side of the load of up to 30A, the cable would handle it perfectly well..

such a fault may be by damage to the insulation of the cable or ingress of water into the FCU's etc.. whatever the cause, the cable would not overheat and catch fire..
exactly the same as it wouldn't if it was a 10A breaker and the fault was 9.9A.. , you've still got extra current than the loads flowing, but it's not damaging the cable or causing a danger..

 

[Goldberg]

It feels like a good time to review the situation.

The 30amp fuse is there to protect the 6mm cable. So need to change.

I'm sincerely hoping that you don't work as an electrician with this approach to selecting circuit protective devices.

I honestly thought that Taylortwocities was in a minority of one, but I hereby concede that I was wrong - it seems that kai and I are in a minority of two.

I apologise for thinking that more of you would understand, but you don't, so I hereby give in.

To summarise what everyone (except kai and myself) believes:

There is no possible risk arising from using an installed cable whose current-carrying capacity, after derating factors, exceeds 30A, for a design load of 8A, with a 30A CPD. No circumstances can possibly arise whereby more than 8A, but less than 30A, passes through any part of the cable, without one of the fuses in the two FCUs breaking, but even if it did happen, which it can't, then it would not be dangerous. All fault modes have been identified and competely addressed in the process of designing and installing the circuit.

 

[ColJack]

I wasn't trying to bend anything, and I still think you haven't answered this question:

1. Imagine a radial circuit with a design load of 5A - let's make it easy and say the conditions make it acceptable to use a type B 6A MCB and 1.0mm² cable.

If I then chose to use 6mm² cable, for my own secret personal private secret private reason, would you then change the CPD to a 40A MCB?

If not, then why not?

if you were designing a circuit from scratch then you select the nearest size breaker for the load and then the cable to suit the installation conditions..

as I've already said..

a NEW circuit designed and installed from scratch, then you put in the closest breaker to the design load and cable size to suit..

so for the third and final time ( and in case you want to hear the actual words.. )
NO, I would not change the OCPD to a 40A just becasue you used 6mm instead of 1.0mm, you design a NEW circuit and fit the nearest size breaker and cable to suit.... but as this is an existing circuit then there is nothing wrong with leaving the 30A fuse in place to protect the 6mm cable as the loads will be connected via FCU's with suitably sized fuses in them..

in tha same way that I wouldn't change a fuse for a cooker circuit if the cooker was no longer used and the socket on the switch was still used for the kettle.. the kettle is protected by the fuse in the plug..

a question to you now..

If you had a 100A supply in a flat that has converted to solar water heating for heat and hot water, and had a gas cooker installed, all of which resulted in a total expected load of 20A comprising a few lights, the telly and the kettle, would you change the main fuse to a 20A fuse or leave it alone and rely on the 6A light breaker and the 20A kitchen ring?

 

[ban-all-sheds]

That's because we prioritise safety above cost.

Why is 6.5 ≤ 10 ≤ 47 safe but 6.5 ≤ 30 ≤ 47 isn't?

Nobody else agrees with you.

You're wrong.

OK - nobody else participating in this topic who has expressed an opinion agrees with you.

In other words, everybody else thinks you have given bad advice.

You're wrong.

OK - everybody else participating in this topic who has expressed an opinion thinks you have given bad advice.

I'd have more success if you simply answered the questions.

No - you would have more success in answering a question if you answered it instead of asking other questions.

Why is 6.5 ≤ 10 ≤ 47 safe but 6.5 ≤ 30 ≤ 47 isn't?

It is the point. It's been the point for many pages and days.

No - it isn't the point.

The point is why MUST you change the OPD to create 6.5 ≤ 10 ≤ 47 instead of leaving 6.5 ≤ 30 ≤ 47?

Cable capacity, in this instance, has nothing to do with kai's proposed reduction in MCB nominal rating.

The requirement is for Ib ≤ In ≤ Iz. If it has nothing to do with cable capacity, it must be to do with load, so why is 6.5 ≤ 10 safe but 6.5 ≤ 30 isn't?

But let me get this straight - BASandmyself is more concerned about the cost than applying some thought about the risks involved in using an overrated breaker?

A breaker rated at or above that of the load and at or below that of the cable capacity is not overrated.

Are you ever going to answer the question why is 6.5 ≤ 10 safe but 6.5 ≤ 30 isn't?

 

[ban-all-sheds]

To summarise what everyone (except kai and myself) believes:

There is no possible risk arising from using an installed cable whose current-carrying capacity, after derating factors, exceeds 30A, for a design load of 8A, with a 30A CPD. No circumstances can possibly arise whereby more than 8A, but less than 30A, passes through any part of the cable, without one of the fuses in the two FCUs breaking, but even if it did happen, which it can't, then it would not be dangerous. All fault modes have been identified and competely addressed in the process of designing and installing the circuit.

If you don't believe that would you care to tell us why you believe that 6.5 ≤ 30 ≤ 47 is not safe but 6.5 ≤ 10 ≤ 47 is?

 

[333rocky333]

I apologise for thinking that more of you would understand, but you don't, so I hereby give in.

Please explain then, I for one am prepared to learn.

I can only assume you want the circuit to disconnect faster in a fault.
Prefered maybe but not a must.

I agree a lower rated mcb is sometimes preferential, I actually have a 10 amp rcbo on 2.5 for a 13 amp spur for my bath pump at home.
This is as I would expect the 10 amp to disconnect faster than the fuse which is behind the bath panels.

However I would still use a minimum of 16 amp at work to feed a 13 amp spur.