Immersion fuse or MCB?

[James]

As James wrote: "In general an mcb (of the right trip characteristics) is better for most applications". dp even better.

Taken out of context.
Please read the full post

Wow, this whole thread is a mass of rubbish about what is better, a fuse or an mcb.

They both have advantages and disadvantages depending upon the load, cable, installation methods and expected use.

In general an mcb (of the right trip characteristics) is better for most applications but there are still plenty of applications that a fuse would be a preferable protective device.

as professional electricians, I cant understand why there is an argument about what is better, they both have there uses and if you choose wrongly then there is a higher likelihood of bad things happening.

get out your books, look at the trip characteristics, expected current, fault current and all the other factors.
make a calculation and come up with the answer.
most of the time it can be done with either one or the other but sometimes one is better than the other.
it cant be decided on a theoretical if this then.... that.

each circuit we install or alter should be given consideration to protective devices and if required, calculation.

 

[westward10]

You say circuit breakers are not one shot devices but what if it's Ics is exceeded?

 

[davesparks]

It's likely a 16A MCB fed from a ring could still comply with all of BS 7671's requirements.

I thought that fused spurs had been limited to 13A now instead of the previous 16A?

DP is superior in many ways as it gives safe isolation, guards against polarity reversal and also isolates neutral to earth faults.

Single pole isolation is acceptable
DP MCBs don't guard against reverse polarity, they do offer protection if a reverse polarity situation was to occur but they don't stop it occurring. If a reverse polarity occurs it is still a fault and a non-compliance.
Yes a DP MCB can isolate a N-E fault if manually switched off, but it won't detect it, some form of RCD would be required for that.

 

[mainline]

Single pole isolation is acceptable

Not in all circumstances

DP MCBs don't guard against reverse polarity,

They guard against getting a shock due to reverse polarity by isolating both N and L

Nobody said it stopped it occurring

Yes a DP MCB can isolate a N-E fault if manually switched off

The question was about whether double pole isolation is better than single pole, and the obvious answer is yes it is.

 

[Steve-S]

I thought that fused spurs had been limited to 13A now instead of the previous 16A?

Single pole isolation is acceptable
DP MCBs don't guard against reverse polarity, they do offer protection if a reverse polarity situation was to occur but they don't stop it occurring. If a reverse polarity occurs it is still a fault and a non-compliance.
Yes a DP MCB can isolate a N-E fault if manually switched off, but it won't detect it, some form of RCD would be required for that.

Double pole RCBOs that sense L&N are superior. They catch N to E faults.

16A can be taken off the final ring when: locally isolated, fused (or MCB) to max 16A and appliance hard wired. Can be a socket but needs to be a Commando socket.

 

[westward10]

I believe all references to isolating the cpc are gone. Much more nonsense and this thread will be gone.

 

[davesparks]

Double pole RCBOs that sense L&N are superior. They catch N to E faults.

Can you define 'catch' please?
All RCBOs sense L and N and will detect N-E faults and operate as a result of them.
So all RCBOs will 'catch' N-E faults, SP RCBOs will operate but not disconnect the fault, SPSN and DP RCBOs will operate and disconnect the fault.

All of which is not relevant to your original point of suggesting that a DP MCB is superior to a SP MCB in a regular domestic installation.

16A can be taken off the final ring when: locally isolated, fused (or MCB) to max 16A and appliance hard wired. Can be a socket but needs to be a Commando socket.

Please can you specify which regulation allows this? As far as I know that regulation has been removed from recent editions and the current regulations only allow accessories to BS1363 to be connected to a ring circuit.

 

[westward10]

It was in the 15th Ed then moved to Guidane Note 1 but I'm not sure if it is still there.

 

[davesparks]

Not in all circumstances

Of course not, but I thought that went without saying. In the average UK installation on the public supply single pole isolation is acceptable and safe.

They guard against getting a shock due to reverse polarity by isolating both N and L

Nobody said it stopped it occurring

The question was about whether double pole isolation is better than single pole, and the obvious answer is yes it is.

No, the question was around the OPs statement that DP MCBs offer superior protection to SP MCBs.

Obvious why? In the average UK installation, where the neutral is reliably and solidly connected to earth at source, there is little advantage to DP over SP isolation.
I'd wager that the majority of DP isolators are installed purely due to that being the only kind available rather than any technical requirement to break the neutral.

 

[davesparks]

It was in the 15th Ed then moved to Guidane Note 1 but I'm not sure if it is still there.

It was in the 16th edition that a fused spur could be up to 16A, I remember coming across it as an apprentice.

 

[westward10]

It was in the 16th edition that a fused spur could be up to 16A, I remember coming across it as an apprentice.

I'll have a gander in a bit.

 

[timhoward]

It was in the 16th edition that a fused spur could be up to 16A, I remember coming across it as an apprentice.

I believe it says that accessories to bs1363 can be connected to a ring final circuit that can contain fused or unfused spurs as long as the current in any part of circuit is unlikely to exceed 20amps for long periods.

Wild paraphrasing from memory as I’m on the beach. This thread is very strange.

 

[Steve-S]

en

I believe it says that accessories to bs1363 can be connected to a ring final circuit that can contain fused or unfused spurs as long as the current in any part of circuit is unlikely to exceed 20amps for long periods.

Wild paraphrasing from memory as I’m on the beach. This thread is very strange.

OSG, App. H2 - final circuits using socket-outlets complying with BS 1363-2 and fused connection units complying with BS 1363-4.

 

[westward10]

It was in the 16th edition that a fused spur could be up to 16A, I remember coming across it as an apprentice.

Appendix 5 (Standard Circuit Arrangements) of the 15th Ed makes reference to 16A devices for permanently fixed equipment.
This Appendix disappears from the 16th Ed and as yet there is no Appendix 15 for ring final and radial circuits in fact the first edition barely makes note of ring final circuits except cpc requirements. Later editions introduce Regulation 433.02.04 which we are now familiar with.
Appendix 15 (Ring and Radial Final Circuits) appears in the 17th Ed which we are still familiar with. No mention of 16A devices.
I know early editions of the Guidance Notes make reference to 16A devices as worded in the 15th Ed but not sure on latter versions.
From what I can see it was never incorporated into a Regulation but was in an Appendix and the Guidance Notes.

 

[westward10]

View attachment 117612
OSG, App. H2 - final circuits using socket-outlets complying with BS 1363-2 and fused connection units complying with BS 1363-4.

Now read H5 which makes this thread pointless.