Fittings rated for continuous duty?

[vitoboy]

Hi all
So had some issues lately related to high current fittings .. dp switches mostly.
A number of electric boilers of various ratings .. from 7kw on up burning out their isolators .. the last one this week was a 7.5kw electric boiler running on a 6mm t&e cable 4m long from a 32A MCB .. 50A dp switch isolator.
No sign of cable overheating at the boiler (even though the terminal block is by the output pipe) . No sign of cable over heating at the consumer unit.. no tripping of the 32A mcb ..
So I understand the the cable is rated to carry over the 32.5 ish A which is max load .. mostly the boiler is modulating and no using anywhere near full power.
Now i know that there are issues with various 13A sockets that are not rated to supply 13A continuously 24 7 .. and these have caused problems with things like car charging.. we use all Hager fittings which I believe are all rated for continuous duty of there rated current... but many manufacturers sockets are not .. my question is are many dp switches and other fittings not rated for continuous duty at their started rating.. and this is causing them to fail at much lower loadings .. and if so how do I find out the continuous duty rating of such fittings.. I believe they should be published ..but a quick search tonight I can't turn up any ratings.
I know at the installation stage it's easy to over rate these things .. which I guess is why the installer for the 7.5kw boiler fitted a 50A dp switch .. but it still failed.

All thoughts greatfully accepted

Ta
Vito

 

[LewisM]

You can contact the Hager support on WhatsApp and ask they normally reply within a day

 

[RichardM]

You mentioned the isolator failed but then referred to it as a switch. An isolator is not designed as a switch, I know it does the same kind of job but it is not designed for continuous on/off usage. An isolator is used to isolate an item of equipment from the supply (for maintenance, long period outage etc. ). They will (eventually) fail if used to disconnect/connect an item under load due to arcing.

 

[Lucien Nunes]

There is a difference between duty cycle and ultimate endurance. A device with limited duty cycle will overheat the first time it is exceeded. E.g. a tool transformer with an intermittent rating will, if loaded to that rating, keep on getting hotter until it is significantly overheated and possibly damaged.

Most pieces of switchgear should be able to withstand their rated load continuously. I.e, if a 20A switch is loaded to 20A it will not continue getting hotter to the point that it overheats. The problem instead is endurance, for which the testing requirements in the BS are probably not sufficient to ensure all products offer the kind of endurance we would like to expect. For example, a BS1363 13A socket outlet must be tested with 14A load on one (to stress it) and 6A on the other (to balance the total to a representative figure). It must stabilise at a certain temperature rise that will not compromise its operation or safety for the duration of the test. But, crucially, it does not say that the socket must be able to do this at ten years old, or even one year old.

That means it's down to the manufacturers and different products might have different test regimes applied. To be sure of what a product can be expected to withstand, I would consult the manufacturers and make no assumptions. For example, I suspect that occasional opening and closing of the switch or isolator will dislodge accumulated oxide and sulphide from parts of the contact surfaces that are not in gas-tight contact, that eventually grows and starts to relieve some of the applied force from the current-carrying asperities. So an endurance test that specifies switching off and on once every six months might not represent the conditions where that never ever happens, trivial though the difference might seem.

 

[vitoboy]

You mentioned the isolator failed but then referred to it as a switch. An isolator is not designed as a switch, I know it does the same kind of job but it is not designed for continuous on/off usage. An isolator is used to isolate an item of equipment from the supply (for maintenance, long period outage etc. ). They will (eventually) fail if used to disconnect/connect an item under load due to arcing.

Hi Richard
Yes .. this had not been used to disconnect at all as far as we can tell .. never been used since installation in fact.
We have always advised customers not to use there shower isolator to switch the shower on and off on load for this reason.
I refer to the isolator as a switch because I'm typing a lot and lazy.!.

 

[RichardM]

Hi Richard
Yes .. this had not been used to disconnect at all as far as we can tell .. never been used since installation in fact.
We have always advised customers not to use there shower isolator to switch the shower on and off on load for this reason.
I refer to the isolator as a switch because I'm typing a lot and lazy.!.

One thing I didn't mention and that is decreasing quality. I have wired many switches over the years, decades, feels like bloody eons and the quality of fittings has definitely gone down hill, just about everything appears to be made in China?
I am also a fan of Hager but not for their shower isolators. I prefer the Crabtree, they are easier to fit giving a little more room to maneuver 10mm cable into position. They also have a deeper feed in for the cable & if I remember correctly 2 securing screws per 'pin'. I would always prefer to fit a wall mounted isolator but few customers will (understandably) wear the additional cost of chasing, plastering etc.

 

[timhoward]

I prefer the Crabtree, they are easier to fit giving a little more room to maneuver 10mm cable into position. They also have a deeper feed in for the cable & if I remember correctly 2 securing screws per 'pin'. I would always prefer to fit a wall mounted isolator but few customers will (understandably) wear the additional cost of chasing, plastering etc.

I agree, Crabtree are the best. A further handy tip is that the Mk 50A isolator and Crabtree screwdriver positions are different by 90 degrees.
If some muppet has fitted either of them in a certain orientation right next to a wall it is almost impossible to tighten the back screws and it's always better to swap to the other brand.

 

[Lucien Nunes]

I don't fit showers or cookers so I haven't looked at switch offerings recently, but are there any where the switch mechanism fits into the box first and then a cover fits over the top, gridswitch style, like some of the 1960s / 70s types? Thinking back to the traditional MK portrait 2G cooker switch, you would put the cables into the terminals, seat the switch mechanism and then tighten from the front, so that once tightened it wouldn't be disturbed.

 

[Eddy Currents]

One thing I didn't mention and that is decreasing quality.

This is sometimes intentional and is called planned obsolescence, you don't see it as much with electrical accessories due to the safety aspect but some countries aren't as concerned with safety but either way they can still reduce quality even if keeping them safe

There is an interesting video which shows the history of the light bulb and the "Phoebus Cartel" where all manufacturers got together to reduce the lifespan of light bulbs to increase profits...

 

[vitoboy]

I don't fit showers or cookers so I haven't looked at switch offerings recently, but are there any where the switch mechanism fits into the box first and then a cover fits over the top, gridswitch style, like some of the 1960s / 70s types? Thinking back to the traditional MK portrait 2G cooker switch, you would put the cables into the terminals, seat the switch mechanism and then tighten from the front, so that once tightened it wouldn't be disturbed.

Nope

 

[vitoboy]

This is sometimes intentional and is called planned obsolescence, you don't see it as much with electrical accessories due to the safety aspect but some countries aren't as concerned with safety but either way they can still reduce quality even if keeping them safe

There is an interesting video which shows the history of the light bulb and the "Phoebus Cartel" where all manufacturers got together to reduce the lifespan of light bulbs to increase profits...

I don't think there is a conspiracy here .. but I do think 5he sort of loadings we are talking about with things like electric boilers and single phase car chargers have just never been seen before .. I was looking at a 14kw electric boiler today ..so 14000÷230=60.8A .. but the paperwork with it says must be installed with an 80A circuit.
If its a holiday house the tenant may well turn the heating on max for days .. after all its free for them.
Hell.. that's the size of a house supply these days .. let alone a flat at 60A probably.
These boilers are used where there is no gas .. so the property will likely be cooking on electricity also .. put on a kettle .. toaster .. boiling water tap ..coffee machine .. times that by 8 or 10 flats in a building even if they are on 80A supply and soon things are going ---- up so to speak..