Heater start up currents

[buttonmoon]

Hi fellas

I've got an outdoor heating circuit that I'm trying to fault find.

There are four heaters under a parasol each rated at 1.5kW that are tripping two MCBs in series. One is on the main board the other is in a separate unit dedicated to the outdoor circuit. When the heaters are cold if all four heaters are plugged in then both Mcbs trip instantly. If only two are plugged in it starts and then you can plug the other two in and there's no problem. Then when it times out if it's switched back on immediately then they come on without a problem.

Both MCBs are B types BSEN 60898 hager and MK.
There is no problem with IR
The measured resistance of a heater when cold is 4 ohms
This would give each one a starting current of 57.5 A
So the four together would have a startup current of 230 A or 52 kW

My first thought was to change the MCBs to C or D types but on reflection of these figures that would just result in a tripped main switch. I am thinking of some sort of soft start current control option but I haven't used one before and I'm not sure if that's the best solution.

Apparently it used to work, but I don't see how.

All thoughts welcome.

 

[Grey Master]

Is it possible to put two or more switches in for the heaters so that not all can be powered up at the same time i.e. adding an extra contactor and then soft starters?
Or individual switches for each heater, there are touch switches with timers espescially for heaters out there.

 

[Guest55]

given me a real good chuckle this thread.
carry on lol.

 

[Guest55]

£300 of relays , contactors & soft start controls and it turns out to be a duff element
oh the look on the customers face - priceless
;-D

will someone please stop me from trolling this thread - the possibilities are endless lol.

 

[GLENNSPARK]

well....we`v all been following the `buttonmoon` thread....
and so its over to Mr spoon....

 

[Knobhead]

I'll PM him.

 

[GLENNSPARK]

well we`l probably be treated to visions of umbrellas floatin through space in a bit...
anyone emember `big bottle`....and `little bottle`?

 

[Deleted member 9648]

"Put a type C or D in."

I tried your idea last night Tony and the main switch went.

I wasn't there when the main trip went but as this is a dedicated circuit and they told me that some other equipment went at the same time I have to assume it was the main switch. which was a 100A double pole main switch on a Proteus board. The reason it tripped is that with the additional current of 4 x 57A, albeit temporarily, added to the rest of the load, on a busy Friday night, it had been tripping the 32B breakers, the 32Cs that I installed allowed the current but the main switch didn't.

Ok We'll start again. The only time this new 'device' tripped is when I wasn't there after I changed the MCBs which were tripping two C types. There is an RCD on the dedicated board which hasn't tripped at any point. Fair point about the main switch thing but it's academic really because if it was an earth fault the other RCD should have tripped before any of the MCBs.

Never seen such contradictory information posted on here before.

 

[ipf]

Is it me or is this forum getting infested with people who can't take critisium??

M, do you take that with chips....or is it radioactive?

 

[buttonmoon]

I tried your idea last night Tony and the main switch went.

The thing tripped after I left and they said some other stuff had also stopped working so I assumed it was an over-current protective device controlling more than just this circuit. If it had been an earth fault then I would have expected the RCD to trip When I was there the only thing that was tripping was the two B type MCBs; one on the board and one on a mini-DB with an RCD and a 32B MCB. It was in a very busy pub at 6 in the evening, the landlord didn't have time to speak to me and I didn't really think that I'd fixed anything, so I said over a rammed bar that I thought it would probably keep tripping and to call me when he could or if he had any more problems. He called me as the pub was closing and said it had tripped again so I told him to only plug in two heaters which he'd been doing successfully before I arrived and I'd try to work out what the problem is. Which is what I'm still doing now 4 hours later.


​

 

[buttonmoon]

That's not angry bold that's it won't turn off bold

 

[buttonmoon]

That's four hours after i started this post not four hours after pub closing time for anyone looking for more holes in the story???

 

[GLENNSPARK]

That's four hours after i started this post not four hours after pub closing time for anyone looking for more holes in the story???

where`s Mr spoon?...

 

[Clive P]

Hi,
Big tin hat on nicely polished, just a couple of thoughts.
In your original post you say 1500 Watts and 4 Ohms, which can't be right (or can it ). I'm saying this because I have installed hundreds of 500W Double envelope Halogen lamps in floodlights over the years. These should have a resistance of just over 100 Ohms. If tested cold they are close to a dead short and only fulfil their requirements once operational, ie hot. we protect these with DP 6A RCBO's Type C ( Contactors and individual protection of each lamp )and don't have a problem.
My final thought is, are you using some sort of sub-standard MCB that just wont continue to work with the constant switching of these loads?

Regards.

 

[buttonmoon]

I'm planning to got back today, I've got a battery for my clamp meter and I'm going to plug each unit in on its own and get someone to turn it on while I see what the current does. I could also check the IR after I have turned each one off again to see if there is any change when they are hot although I'm not sure how this will help as the fault only occurs if they are turned on from cold. Hopefully they can tell me precisely what happened when it tripped the last time and some other circuits were affected. Unfortunately the four units are supplied by a 4mm underground SWA so to separate the supplies would involve digging up their patio or installing something at the top of their umbrella.

I have to assume that I've got something wrong otherwise it would never have worked in the first place so if I haven't worked it out through the tests I'm planning above I'm going to redo all of my earlier tests to double check.

"My final thought is, are you using some sort of sub-standard MCB that just wont continue to work with the constant switching of these loads?!"

The MCBs which were in stalled when I arrived were both B32s, a Hager and an MK and both were tripping simultaneously which makes me think that its not likely to be due to the same identical problem with both of them.

Any other tests, ideas or general grilling welcome

 

[electric will]

If The resistance of the elements is 4 ohms while cold this will increase as the element heats up, it's called PTC, causing the current to fall, I be leave this is how elements self regulate or they'd just keep getting hotter and burn out.
maybe the initial current draw is 57 odd amps, the meter and sums don't lie? But it'd only be for a very very short time.
But there must still be a another problem, I take it these feathers can be plugged into a 13 amp socket? People would be tripping there MCB's all over the place