Another Intermittent Fault

[SJD]

By marconi A good question, as ever. My answer: It may be the boiler has a supply derived from the RFC as an FCU in the ring or as a spur off it. And if it is, there is switching taking place in the boiler electrical system as thermostats and timers operate. And, the new boiler/heating system is a major, recent change correlated to the onset of the intermittent fault. As ever - I may be wrong.

The boiler FCU is on the RFC, which I turned off when conducting the IR tests on the circuit alone. But my reasoning is that if the fault took out the 32A MCB, then if on the boiler it ought to take out the FCU 3A fuse as well - and the boiler is still operational with an intact fuse.

 

[marconi]

The boiler FCU is on the RFC, which I turned off when conducting the IR tests on the circuit alone. But my reasoning is that if the fault took out the 32A MCB, then if on the boiler it ought to take out the FCU 3A fuse as well - and the boiler is still operational with an intact fuse.

By Marconi. More information for the brains trust to ponder. Thank you.

 

[marconi]

The boiler FCU is on the RFC, which I turned off when conducting the IR tests on the circuit alone. But my reasoning is that if the fault took out the 32A MCB, then if on the boiler it ought to take out the FCU 3A fuse as well - and the boiler is still operational with an intact fuse.

By Marconi First have you confirmed the size of fuse protecting the boiler circuit?

Second, whether the MCB operates before the cartridge or vice versa depends on their 'Isquaredt' (current x current x time) characteristics. Here is a graph for a 10Amp MCB and 5 Amp fuse I pinched off the internet:

https://www.google.co.uk/search?q=d...KbvUo7z-M:&usg=__l3H6IkxcmtJcsfbwCPTNuI4Z8w4=

When the line for the fuse is to the left of that for the MCB, the fuse ruptures before the MCB trips. Otherwise, the MCB operates first albeit with fault current even more to the right both might/will operate. The crossover of the fuse and McB characteristics takes place at about 100Amps. Remember too that the fuse relies on the thermal effect of the current but the MCB uses both thermal and magnetic effects.

I only know the theory. The practice of selecting overcurrent protection devices to achieve discrimnation was not something I have ever had to do. other commentators can/will no doubt amplify further.

 

[Octopus]

By Marconi First have you confirmed the size of fuse protecting the boiler circuit?

Second, whether the MCB operates before the cartridge or vice versa depends on their 'Isquaredt' (current x current x time) characteristics. Here is a graph for a 10Amp MCB and 5 Amp fuse I pinched off the internet:

https://www.google.co.uk/search?q=d...KbvUo7z-M:&usg=__l3H6IkxcmtJcsfbwCPTNuI4Z8w4=

When the line for the fuse is to the left of that for the MCB, the fuse ruptures before the MCB trips. Otherwise, the MCB operates first albeit with fault current even more to the right both might/will operate. The crossover of the fuse and McB characteristics takes place at about 100Amps. Remember too that the fuse relies on the thermal effect of the current but the MCB uses both thermal and magnetic effects.

I only know the theory. The practice of selecting overcurrent protection devices was not something I have ever had to do. other commentators can/will no doubt amplify further.

So whats your motivation matey - you claim to be a landlord, but you seem to be posting more and more - so are you really a sparky?

 

[marconi]

So whats your motivation matey - you claim to be a landlord, but you seem to be posting more and more - so are you really a sparky?

By Marconi My motivation - an abiding interest in electrical engineering. I spent time in the 1970s working as an electrician. I then joined the Royal Navy. I gained a first class honours degree in engineering specialisng in electronic, electrical, control and real time computing. I served at sea in two frigates, first as the deputy of one of te two engineering departments and later as head. In each case I had 30-40 odd highly trained and qualified technicians working for me. Alone at sea we only had ourselves and our wits to keep complex equipment operational so we had to good at diagnosis of faults, work together as a team and draw on each others knowledge and experience. I am a chartered engineer and chartered electrical engineer.

I will not bore by detailing my other experience save for my interest in electronics - recent projects being the electronics to control 4 sets of traffic lights and another an 8 floor lift.

You will note I only venture to post on certain topics - the wiring regulations and practice of wiring I stay clear of because it beyond my competency to comment. Fault finding is not.

I am fortunate to be retired. The flats are my wife's pensions (she is chronically ill and disabled) - so that in the unfortuante event of my ill-timed demise she has enough income to look after herself. So I am a landlord and do some volumtary work and act as a carer for my wife.

This forum provides some interesting mental stimulation. And I think I can help and I suppose provide some education once in a while.

Am I not welcome in the EF or by you in particular?

 

[PEG]

By Marconi My motivation - an abiding interest in electrical engineering. I spent time in the 1970s working as an electrician. I then joined the Royal Navy. I gained a first class honours degree in engineering specialisng in electronic, electrical, control and real time computing. I served at sea in two frigates, first as the deputy of one of te two engineering departments and later as head. In each case I had 30-40 odd highly trained and qualified technicians working for me. Alone at sea we only had ourselves and our wits to keep complex equipment operational so we had to good at diagnosis of faults, work together as a team and draw on each others knowledge and experience. I am a chartered engineer and chartered electrical engineer.

I will not bore by detailing my other experience save for my interest in electronics - recent projects being the electronics to control 4 sets of traffic lights and another an 8 floor lift.

You will note I only venture to post on certain topics - the wiring regulations and practice of wiring I stay clear of because it beyond my competency to comment. Fault finding is not.

I am fortunate to be retired. The flats are my wife's pensions (she is chronically ill and disabled) - so that in the unfortuante event of my ill-timed demise she has enough income to look after herself. So I am a landlord and do some volumtary work and act as a carer for my wife.

This forum provides some interesting mental stimulation. And I think I can help and I suppose provide some education once in a while.

Am I not welcome in the EF or by you in particular?

...haven't we all....Giggerty :yesnod:

Just breaking the tension,with a comedy cartoon reference...

I think,sometimes,on an anonymous forum,the asking of a question such as previous,allows an answer which can inform many.

Fair play to you,for your candid history explanation. I would be struggle to list mine as succinctly,and remain believable...:cool4:

 

[marconi]

Dear Murdoch: If I knew how to, I would remove the last line of my post #15. It was uncalled for and unnecessarily terse.

 

[andyjthompson]

I a number of years back had a similar fault. The occupant reported a loud bang periodically, but was unable to exactly say where abouts! What I eventually was a water leak from the bath that had found its way to a nail in the floor boards that had pierced a 2.5mm T & E. The bang was the short to earth which never took out the re-wirable fuse.

 

[PEG]

In relation to SJDs' original post,i too,had a similar fault,which i have posted the details of on here somewhere....

It was the same situation,intermittent tripping of MCB,once in a week...or every 5 minutes....IR showed clear,but the inspecting of all sockets,revealed a face plate screw,with a blackened,knobbly end...

I put the variable interval of tripping,to temperature changes,in the conservatory i found the BBQed socket in :sailor:

 

[Octopus]

By Marconi My motivation - an abiding interest in electrical engineering. I spent time in the 1970s working as an electrician. I then joined the Royal Navy. I gained a first class honours degree in engineering specialisng in electronic, electrical, control and real time computing. I served at sea in two frigates, first as the deputy of one of te two engineering departments and later as head. In each case I had 30-40 odd highly trained and qualified technicians working for me. Alone at sea we only had ourselves and our wits to keep complex equipment operational so we had to good at diagnosis of faults, work together as a team and draw on each others knowledge and experience. I am a chartered engineer and chartered electrical engineer.

I will not bore by detailing my other experience save for my interest in electronics - recent projects being the electronics to control 4 sets of traffic lights and another an 8 floor lift.

You will note I only venture to post on certain topics - the wiring regulations and practice of wiring I stay clear of because it beyond my competency to comment. Fault finding is not.

I am fortunate to be retired. The flats are my wife's pensions (she is chronically ill and disabled) - so that in the unfortuante event of my ill-timed demise she has enough income to look after herself. So I am a landlord and do some volumtary work and act as a carer for my wife.

This forum provides some interesting mental stimulation. And I think I can help and I suppose provide some education once in a while.

Am I not welcome in the EF or by you in particular?

No, just curious. You wouldn't be the first person to start off by being a "diyer" and later come out as a sparky in training.

and why do you put "By Marconi"?

Its obvious who the poster is?