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I've had that on a cooker circuit.
Supply size
- Thread starter RF Lighting
- Start date
I've had that on a cooker circuit.
How is that different to installing a new fuse(wire) and then switching on the fault ? Apart, that is, from the holes in the fuse carrier socket
In both cases, you are applying a short circuit (I'm guessing) - just changing where the fault is "switched". Still, I suspect the chap might have needed fresh underwear
It's no different, just telling the story as it happened.....
OK, just sounded like "CU still on" was a material fact - other than the bang being "something of a surprise" to the chap inserting the fuse.
EDIT: Just a thought, has anyone ever had problems with the main switch welding shut if closed onto a short circuit ? I know they are designed to be positive opening (ir the switch toggle forces the contacts apart), but this might not be enough if the contacts are "well stuck".
EDIT: Just a thought, has anyone ever had problems with the main switch welding shut if closed onto a short circuit ? I know they are designed to be positive opening (ir the switch toggle forces the contacts apart), but this might not be enough if the contacts are "well stuck".
That does sound very interesting. I'll have a bit of a play at the weekend and report back
Experience and theory indicate that closing the switch at the zero crossing point creates very little inrush current.
Something like this on zero crossing mode for resistive loads
http://docs-europe.electrocomponents.com/webdocs/0efc/0900766b80efc5ae.pdf
RS Stock No.720-4007
should "warm up" the tranny without taking out the breakers and then once the tranny is running use a mechanical switch to by pass the solid state switch.
The random fire mode for inductive loads is for when the load current is not in phase with the voltage and defeats the zero crossing detector in the switch. On the first cycle the transformer winding is resistive until the flux creates back EMF to oppose the applied voltage. Once the transformer is producing back EMF and is unloaded the solid state switch should survive until the mechanical switch is closed. It may be necessary to disconnect the solid state switch BEFORE opening the mechanical switch when closing down. The back EMF when the mechanical switch opens is very likely to be very large and could take out the solid state switch
Yes The solution is two switches in series. If one switch welds on closure then the other can still be opened to stop the supply to the machine.
Quite often one contact in a double pole switch will weld and go un-noticed as the other contact does the switching and all seems OK.
I think the theory would go something like (description for positive half cycle, same applies for negative half cycle) :
With the supply already applied, the first half of the positive voltage cycle will be killing the negative current in the inductor, the second half of the positive voltage cycle is building up the positive current.
If you connect the supply at the negative-positive zero voltage crossing, the inductor current is zero, and so the whole positive voltage cycle goes towards building up a positive current. The current (and hence magnetic field) will increase past it's normal maximum, the magnetic circuit will saturate, and the effective impedance will drop significantly - causing a large spike in current drawn.
Apply the supply at voltage maximum and the inductor current only increases for the latter half of the voltage waveform. This is exactly the same as an already applied supply where the inductor current will be passing through zero as the voltage reaches maximum.
This will explain why when switching on a transformer, it's more or less random whether it "just turns on" or turns on with a big "thwump", or somewhere in between. It all depends on the point in the voltage cycle when the switch contacts actually close.
Why ? It's just a solid state version of a normal switch - it won't limit the current, and in zero crossing mode will guarantee to create the highest inrush current. A random switch on version will statistically sometimes switch on at (or close to) voltage zero crossing which will produce the same problem.
I think that's the wrong way round. A transformer winding is inductive unless you saturate the core, at which point the effective inductance drops off. This is very different to a motor where you are relying on back EMF from the inherent generator to reduce the current once the rotor is turning.
Did a quick search, and the first result was this :
http://www.te.com/commerce/Document...v&DocNm=13C3206_AppNote&DocType=CS&DocLang=EN
(It's linked to from this page, but ever so helpfully
they've made it so it just downloads the doc which together with Google's deliberate link obfuscation makes it fiddly to find the real link).The first paragraph from that document says :
A zero-crossover solid-state relay may be the worst possible method of switching on a transformer or a highly inductive load. Evidence1 has come to light that zero-crossover turn-on of such loads can cause a surge current of perhaps 10 to 40 times the steady state current, whereas turn-on at peak voltage results in little or no surge.
Turn off of an inductive load with an SSS shouldn't be a problem, as most use a triac which won't turn off until the current is around zero - so should produce minimal voltage surge in the load. If something external happens (supply trips for instance) then it would be different !
DNOs must face this problem in a very big way . I wonder what happens when they power up one of their mega transformers - do they just 'accept' the very high 'inrush' current, or do they take some measures to mitigate it?
Kind Regards, John
Kind Regards, John
Yes, I suspect it's a problem, but they will also deal with higher capacity supplies - so I assume less problems that we face trying to energise a transformer who's FLC is higher than the rating of the breaker supplying it.
They also face a bigger problem turning stuff off. A long line has significant capacitance as well as inductance, so if you get "the wrong sort of arcing", very rapidly you can get voltage surges several times the rated peak voltage of the line.
See some examples at http://www.capturedlightning.com/frames/longarc.htm
(items 2, 3, 6 are most relevant)
They also face a bigger problem turning stuff off. A long line has significant capacitance as well as inductance, so if you get "the wrong sort of arcing", very rapidly you can get voltage surges several times the rated peak voltage of the line.
See some examples at http://www.capturedlightning.com/frames/longarc.htm
(items 2, 3, 6 are most relevant)
Big bloody resistor and a relay to bypass it after a time delay.
OUCH
I was wrong. I must not trust my memory again when the matter is not a simple matter.
I was wrong. I must not trust my memory again when the matter is not a simple matter.
Going to try a couple of different sized heaters first and see if that works as I can cobble it together out of stuff I've knocking around the workshop.
That does sound very interesting. I'll have a bit of a play at the weekend and report back
When you get bored of that, try the same trick with a switch start fitting, with the shorting switch wired across the choke, quite spectacular!
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