Jump to content
 

SHORT CIRCUIT PROTECTION


TEAMYAKIMA
 Share

Recommended Posts

Hello

 

I have a power box full of transformers. The power box was made for me by a friend/expert.

 

Unfortunately he did not put any short circuit protection in the power box.

 

Today a short circuit on my 16v AC bus nearly burnt out one of my transformers - lots of smoke but managed to get it unplugged in time!

 

What is the 21st century solution to this problem ? Something that auto re-sets hopefully.

 

I have an RS components just down the road so an RS components solution would be ideal.

 

Thanks

 

Paul

Link to post
Share on other sites

If you want something you can reset, search for circuit breaker.

The voltage rating is an upper limit. It is the current which trips it.

 

If you want something cheaper & more simple, then a fuse. It may be a low-tech solution but there is nothing wrong with that. The only drawback is you need to replace it after you have a fault.

Link to post
Share on other sites

I'd stop using this power box until it is fully checked out. A possible fire or shock is never worth the risk, count yourself very lucky you got away with it this time, you may not be so lucky next time.

For overload protection, as a minimum, each transformer would need its own fuse of a suitable rating on the input (mains) side as well as possibly one on the output too (a poly fuse may be suitable on the low voltage side in addition to the one on the mains side). You should NOT have one fuse on the mains side set to the maximum of all the transformers put together as if only one draws too much current on its own it will not blow the single fuse and possibly overheat as a result.

I believe exhibition organisers are becoming cautious on allowing 'home made' mains circuits to be used for the very reason you have found.

The above does not save you from needing double isolation / adequate earthing which is another thing to be aware of. Without seeing the power box in person I don't think anyone can give you a definitive solution.

If in any doubt don't risk it with mains, you may not get a second chance.

  • Agree 5
Link to post
Share on other sites

  • RMweb Premium

More and more exhibitions are now insisting on PAT testing anything electrical, and anything failing a test can be deactivated (removal of plugs and/or leads). I saw a soldering iron offered for PAT testing where the outer insulation of the cable had pulled away from the plug, and just the individual red and black wires could be seen entering the plug. It did not pass the test.

 

Home brewed transformers in wooden boxes or plastic tool boxes are not looked on favourably these days. Genuine Switch Mode Power Supplies (SMPS) as used by laptops are superior, safer and lighter, but some cheap ones from the far east can be fire hazards, so beware.

 

Link to post
Share on other sites

  • RMweb Premium
3 hours ago, Ian Morgan said:

More and more exhibitions are now insisting on PAT testing anything electrical, and anything failing a test can be deactivated (removal of plugs and/or leads). I saw a soldering iron offered for PAT testing where the outer insulation of the cable had pulled away from the plug, and just the individual red and black wires could be seen entering the plug. It did not pass the test.

 

Home brewed transformers in wooden boxes or plastic tool boxes are not looked on favourably these days. Genuine Switch Mode Power Supplies (SMPS) as used by laptops are superior, safer and lighter, but some cheap ones from the far east can be fire hazards, so beware.

 

Items intended for use in a public location, such as an exhibition, should be tested in advance. Nothing more embarrassing (and quite possibly rude words from them), to an owner on finding out that the exhibit, is unacceptable on the day!

 

Some people still think that what they do in their own homes, is their business. But it is very much a case of not worth the risk, as you are dead for a very long time!

Link to post
Share on other sites

As you say you want auto-reset, the choice is basically down to the appropriate rating "polyswitch" style PTC thermal fuse.

 

They switch to high resistance when overloaded but return to normal in a few seconds, after the overload is removed or power switched off.

 

See here: https://uk.rs-online.com/web/c/fuses-sockets-circuit-breakers/fuses-pcb/resettable-wire-ended-fuses/

 

Select parts to suit the current ratings of each transformer secondary winding.

The "trip" current should be about the same, no more than say 10% higher, than the transformer secondary continuous rated current.

 

Connect an appropriate one in series with each transformer secondary winding.

 

It's also a good idea to put a conventional 240V AC (or higher) rated HRC cartridge fuse in line with each transformer primary, rated at something like 2 - 4 times the maximum continuous primary current of that specific transformer.

You need to allow a margin on those as a transformer can momentarily take a rather higher than normal current at the instant power is switched on & an exact rated fuse can blow for no good reason.

 

If primary current is not given, divide the VA rating (or  wattage, if VA is not given) of the transformer by the nominal mains voltage to get the maximum input current.

 

Individual primary fuses allow a fault in any one transformer to blow its own low rated fuse, rather than needing to be so serious it must take out the plug fuse feeding the whole system - which is often left as a 13A one even is a 3A or 5A would be adequate...

 

Link to post
Share on other sites

  • RMweb Gold

I would definitely replace the affected transformer because that “lots of smoke” was the windings insulation being destroyed and any amount of protection (on the Secondary side) will not protect a short within the transformer itself.

 

So “managed to get it unplugged in time!“ is not true.

 

 

Kev.

  • Agree 5
Link to post
Share on other sites

Controllers these days seem to use polyswitches, they cost virtually nothing (see screenshot 13.4 p each) and fitted in series with the load will gradually cut the power if overloaded.  They are a bit too slow acting to save some fragile components but fine for protecting transformer wiring, but hey when you are only paying £50 for a controller you can't expect top quality components.  I would pop a 1 amp one in every DCC feed wire and maybe a 0.5 amp in every DC section feed wire. 

Older controllers back in the 1950s  etc had decent overload protection, the best ones like Triang had to be reset by hand. Old H/D ones had an auto reset and a warning bulb these protect the armature windings on old H/D locos etc which modern units don't do. The old H/D Marshall 3 will growl and flicker the warning lights right on their 1 amp limit.  There are manually reset able circuit breakers available but although I have several they are not in model railway ratings so I can't be sure how far above their rating they actually trip, and how quickly. which is kind of crucial to protect point tangs which can only carry 1 amp without suffering localised burning and some loco parts. 

If the transformer had smoke pouring out of it and has gone a sort of crispy black colour it may well be worth binning it before it electrocutes you or burns the shed down.

 

Screenshot (9).png

Edited by DavidCBroad
  • Agree 1
Link to post
Share on other sites

  • RMweb Gold
On 13/08/2019 at 16:43, TEAMYAKIMA said:

Hello

 

I have a power box full of transformers. The power box was made for me by a friend/expert.

 

Unfortunately he did not put any short circuit protection in the power box

 

If your friend really was an expert there would be proper protection of your power box, and more importantly, proper protection of any human being including yourself.

 

Personally I would not be trying to work out how to sort out this box at all. I would be sourcing a proper factory built power supply.  Well you did ask for the 21st  century solution.  Home built power supplies are so  1950s .   

 

Be safe.

Link to post
Share on other sites

  • RMweb Premium
2 hours ago, Colin_McLeod said:

 

If your friend really was an expert there would be proper protection of your power box, and more importantly, proper protection of any human being including yourself.

 

Personally I would not be trying to work out how to sort out this box at all. I would be sourcing a proper factory built power supply.  Well you did ask for the 21st  century solution.  Home built power supplies are so  1950s .   

 

Be safe.

Agreed modern equipment is much safer than that of old.

 

A recent example. I was with a mate of mine and he complained that the portable electric heater was no good.

I glanced at it and told him, ever helpfully, that it was working as it should!

But there's no heat coming from it, he said.

So I picked it up and put it down again and voila, heat from it.

The difference was, this time it was level and the safety switch was pushed in, thus completing the circuit.

 

Explained to him that in the good old days, it may have fallen over and started a fire!

  • Like 2
Link to post
Share on other sites

Hi

I use these (Gaugemaster also use then on their DC controllers). They are sold in various current ratings usually from 1.0Amp up -  Search web site for other current ratings.

1.0Amp Self resetting thermal CB

1.6Amp Self resetting thermal CB

One CB is wired in series with one output feed from each transformers secondary output.   Ensure you obtain the correct current rating for the transformer.

 

As suggested by others, do get the power supply tested and inspected by a qualified electrician though BEFORE attempting any further use! 

Link to post
Share on other sites

3 hours ago, Crosland said:

 

I think a fair few manufacturers would take offence at that comment.

Bitter experience.   Polyswitches are great for manufacturers as they protect the controller from overload at extremely low prices.  They also fail and you scrap an otherwise perfectly good controller because these days no one wants to drill out the rivets and solder a new 13p component in place.  However this is not good for our trains. In my experience  Magnetic overloads on the old 1950s style controllers are much more sensitive than thermals but I can't find any magnetics in the 1 amp range at a reasonable price, (On Ebay and on a Google search) something like £16 was the least and that was 28 volt, Thats 100 times the cost of a polyswitch.  Hence my comment.  

If anyone knows of a decent magnetic cut out round 1 amp for 12 - 20 volts DC I would love to know.

Link to post
Share on other sites

  • RMweb Premium
29 minutes ago, DavidCBroad said:

Bitter experience.   Polyswitches are great for manufacturers as they protect the controller from overload at extremely low prices.  They also fail and you scrap an otherwise perfectly good controller because these days no one wants to drill out the rivets and solder a new 13p component in place.  However this is not good for our trains. In my experience  Magnetic overloads on the old 1950s style controllers are much more sensitive than thermals but I can't find any magnetics in the 1 amp range at a reasonable price, (On Ebay and on a Google search) something like £16 was the least and that was 28 volt, Thats 100 times the cost of a polyswitch.  Hence my comment.  

If anyone knows of a decent magnetic cut out round 1 amp for 12 - 20 volts DC I would love to know.

Even H&M didn't supply 1 amp cutouts, magnetic or thermal. They made magnetic ones CT15, CT30 & CT40 being 1 1/2, 3 and 4 Amp.

The thermal ones were the same values.

 

Info from an old H&M catalogue.

 

Modern motors would probably die long before those cutouts worked! They were designed for X04's and Hornby Dublo motors.

Link to post
Share on other sites

50 minutes ago, DavidCBroad said:

Bitter experience.   Polyswitches are great for manufacturers as they protect the controller from overload at extremely low prices.  They also fail and you scrap an otherwise perfectly good controller because these days no one wants to drill out the rivets and solder a new 13p component in place. 

 

The warning on Littelfuse's (and other manufacturers') datasheets states:

 

These devices are intended for protection against damage caused by occasional overcurrent or overtemperature fault conditions and should not be used when repeated fault conditions or prolonged trip events are anticipated.

 

If you're getting more than occasional overcurrent conditions that are causing them to fail you're probably doing something wrong further down the line. 

Edited by sharris
  • Agree 1
Link to post
Share on other sites

  • RMweb Premium
32 minutes ago, sharris said:

 

The warning on Littelfuse's (and other manufacturers') datasheets states:

 

These devices are intended for protection against damage caused by occasional overcurrent or overtemperature fault conditions and should not be used when repeated fault conditions or prolonged trip events are anticipated.

 

If you're getting more than occasional overcurrent conditions that are causing them to fail you're probably doing something wrong further down the line. 

Yes, but we have all read previously, of David's experience of 'glowing fishplates'...

  • Like 1
Link to post
Share on other sites

Magnetic mechanisms are complex and expensive, compared to thermal ones.

 

A 1A type B domestic is the cheapest I can find - eg.

https://www.cef.co.uk/catalogue/products/4392033-1a-sp-10ka-type-b-mcb

 

That general type are made in ratings from 0.5A

The "B" type has a magnetic trip point at 4x rated current; the "C" type at 6x rated current.

 

 

A cheaper option is "roll your own" - use a 12V relay wired as self-latching with a sensitive magnetic reed switch as the "on" contact.

eg. These from RS are rated at six to sixteen ampere-turns to activate:

https://uk.rs-online.com/web/p/reed-switches/9092572/

 

With a circuit something like this, the "trigger" being the reed switch. Add a flyback diode across the relay coil to prevent the reed contacts from arcing.
 

024_03.gif

 

Put say, ten turns of insulated wire around the reed switch and it will operate at somewhere around one amp.

 

Use a separate normally closed contact on the relay to disconnect power in the monitored circuit. A break contact button in the relay power circuit will reset the latch.

 

If the trip point is too sensitive, just take a turn at a time off the reed switch - or add turns if it's not sensitive enough.

 

You should be able to make very precise low-current "breakers" for something like two to four pounds each.

 

Edited by RobjUK
typo
  • Like 1
  • Craftsmanship/clever 1
Link to post
Share on other sites

On 15/08/2019 at 12:33, RobjUK said:

Magnetic mechanisms are complex and expensive, compared to thermal ones.

 

 

The simplest thermal version is a PTC (Positive Thermal Coefficient) thermistor, also known as a light bulb. Hornby-Dublo used them in their controllers for years. Twelve volt car brake/tail light bulbs make quite good PTCs. Depending on the transformer you might have to experiment with various parallel combinations of different wattage bulbs to set the right current limit.

 

Light bulbs also have the advantage that they get quite bright when the supply is overloaded. Also, they work equally well on AC and DC.

Link to post
Share on other sites

  • RMweb Premium
36 minutes ago, AndyID said:

 

The simplest thermal version is a PTC (Positive Thermal Coefficient) thermistor, also known as a light bulb. Hornby-Dublo used them in their controllers for years. Twelve volt car brake/tail light bulbs make quite good PTCs. Depending on the transformer you might have to experiment with various parallel combinations of different wattage bulbs to set the right current limit.

 

Light bulbs also have the advantage that they get quite bright when the supply is overloaded. Also, they work equally well on AC and DC.

Good idea and they can be had for free!

 

Many brake/tail lights fail with one filament dying (usually the brake light half) and so useless for cars. So you could recycle the good part of a typical failed 21/5 watt bulb. Add as many in parallel as required.

Link to post
Share on other sites

It might be useful if you listed all of the outputs that this smoky box provided - and with what current ratings !!!

Suitable advice might then be possibe

 

Some of your downstream devices may require ac; others may be 'universal' ... in which case a DC output SMPS will be available.

SMPS are also lighter in weight than 50Hz/60Hz transformers - as well as being more energy efficient, and cheaper to buy.

In any replacement 'box'    ( I recommend plastic boxes from a Really Useful box company - to CARRY the power supplies in - NOT to USE them in 8-)

consider having suitable protection and switches for EACH of the power supplies ... and keeping them separate allows individual replacement in the event of a fault - and individual PAT testing.

  • Agree 1
Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

×
×
  • Create New...