Soak Timer

[gman76]

Hi everybody,

I work for a hi-fi company service department, when work has been done to units, they are put on an overnight "soak" period, where the units are switched on a 10mins on - 5mins off cycle, the idea being that any faults will become apparent during this time.

There are 4 contactors, electronically controlled to time the 4 separate "drops" of mains leads (maximum 6 drops per contactor). The problem we have is, after a particularly busy day, sometimes there are enough large toroidal transformers on a particular drop, that, along with an already energised drop, the inrush current is enough to trip the MCB covering the entire setup. (40a type C).

Due to the constraints of the cabling, uprating the MCB isn't an option at present, so, it would be very handy to know what time the MCB tripped, to ascertain whether the units on soak have had sufficient hours.

My obvious thought was to connect an old analogue electric clock to the input feed, so the time it stops would be apparent.

Just wondered if anyone had any more scientific ideas?

Hope that makes sense & sorry for the long post!

 

[breezer]

My obvious thought was to connect an old analogue electric clock to the input feed, so the time it stops would be apparent.

Just wondered if anyone had any more scientific ideas?

why make it more complicated?

 

[gman76]

Fair comment!

Might look a bit naff though.

Thanks Breezer.

 

[sm1thson]

a cure to stop the MCBs tripping out without doing a big rewire would be to set up some relays/ contactors with delays so when the cycle starts each transformer (or group) can energise before the next ones power up.

 

[Pensdown]

I would have thought that solving the trip problem would be a better solution than knowing when it went off.

You could try the following

• Subject to the circuit being suitable – change the C type MCB for a D type
• Split each drop by adding another contactor. Wire the first contactor as normal and take the feed to the second contactor through a pneumatic delay timer mounted on the first contactor. That would ½ the inrush per drop.

 

[ban-all-sheds]

Subject to the circuit being suitable – change the C type MCB for a D type

Or a fuse?

A thought occurs to me though - is it wise to have possibly faulty equipment being power-cycled with nobody in attendance?

 

[ban-all-sheds]

Might look a bit naff though.

Use one of those plug-in timers then - it'll do the same but not look so naff.

 

[toasty]

I used a NTC thermistor on my Power Amp to limit inrush as it was occasionally blowing a C32 on startup.

Maybe you could wire a Negative temp coefficient thermistor in series with each drop, that'd limit the inrush current nicely.

Something like this:
http://www.ametherm.com/Inrush_Current/Bigamp_datasheet.htm


Hope this helps.

 

[gman76]

The whole timer arrangement is due to be redesigned anyway, which will include staggering the drops so as to not be on together. However, that might not happen in the near future, due to work load. So I was looking for a handy intermediate "fix".

I considered the mechanical plug in timer after posting - which is what I'll go for - nice and easy!

I kind of agree BAS, about switching equipment overnight, however, over the last 35 years, the closest thing to a fire we have experienced was a tuner, luckily on metal shelving, which just made some large scorch marks. That was caused by a lightning strike.

Thanks to all who contributed.