Exposed over door contact

[Hulmerous]

Hi guys,could do with some advice on a matter I came across today on a job. It's to do with a over door contact that is installed from a bell chime unit with built in transformer. The door contact is the two metal strip kind so when the door opens the metal strips connect and sound them chime. My question is those metal strips are exposed to touch. The chime transformer output is 12 volt 1amp. Is that safe? I was always told that anything over 10ma is dangerous. Can someone guide me on this please?

 

[Richard Burns]

Minimum noticeable touch voltage is 25V, so a 12V circuit does not need to be insulated to prevent electric shock.

It may need insulation in places to prevent short circuits but I would say this is not ideal but in no way unsafe.

 

[Hulmerous]

Thanks Richard for your reply. Is it not the amount of amps and not voltage that has to be considered?

 

[Richard Burns]

Amps kill volts thrill!:biggrin5:

It is the amps that can kill you but you will not experience a high current flow through the body (1000ohms resistance) from 12V. Think, you can touch the two contacts on a car battery safely but a car battery can provide hundreds of amps.
If you consider the protective measure SELV then there is no requirement for insulation because you cannot sense that level of voltage. In the case of a bell transformer this is unlikely to be separated so has the potential to become dangerous in case of a fault, but only because of higher voltage transfer.

There is a you tube video somewhere of someone transforming down the supply to a very low voltage but massive current and electrically contacting the output directly without harm. ( I would not recommend this though!)

 

[Hulmerous]

Thanks again Richard.

 

[Geordie Spark]

Hi guys,could do with some advice on a matter I came across today on a job. It's to do with a over door contact that is installed from a bell chime unit with built in transformer. The door contact is the two metal strip kind so when the door opens the metal strips connect and sound them chime. My question is those metal strips are exposed to touch. The chime transformer output is 12 volt 1amp. Is that safe? I was always told that anything over 10ma is dangerous. Can someone guide me on this please?

Did you never play with train sets or Scalextric racing cars when you were younger?

 

[Knobhead]

OH god NO!

The HSE will ban our train sets next!

 

[chrish_1977]

Is it not dc output from transformer?

 

[telectrix]

nope. transformers output is a.c. to get d.c. from a tranny, it needs to be rectified.

 

[tazz]

Come on lads this is basic ohms law.....votage (applied) / resistance (the body) = A (current be passed)
The current is watch is being passed through the body, because this will be in Micro Amps we call this SELV (safe extra low voltage)

 

[Robotstar5]

OH god NO!

The HSE will ban our train sets next!

They might have banned the first ones, they were designed to have the track connected across the mains!

 

[Geordie Spark]

OH god NO!

The HSE will ban our train sets next!

I'm surprised they haven't already.

Best keep quiet about it - we don't want to give them any more stupid ideas!!

 

[Geordie Spark]

They might have banned the first ones, they were designed to have the track connected across the mains!

I can think of a few folk I'd like to let play with that train set !!!

 

[telectrix]

mine's clockwork.

 

[Geordie Spark]

mine's clockwork.

I have both a clockwork AND an electric one so there !!!!

(and a couple of live steam locos I build myself)

 

[telectrix]

and i thought fred dibnah was dead.

 

[cobrauk]

I think you will find that all chime transformers are all DC no AC

 

[snowhead]

I think you will find that all chime transformers are all DC no AC

There is ambiguity in that statement.

If it said all chime power supplies are D.C, it wouldn't be ambiguous but it would be wrong.

 

[SPARTYKUS]

Tazz



have you picked up screwfix syndrome.... Are you sure its "safe extra low voltage"

(I am being picky and feel free to tell me to naff off!)

sparty

 

[SPARTYKUS]

A level physics. 1985. Top teacher (ex RAF engineer). Demo'd this beautifully.

he rigged up a...well a rig at the front with thousands of windings versus well, two or three about a centimetre thick (or a centimeter if you're American) with huge clamps either end. Got a monster nail out (all to prove the amps point), asked what we thought would happen etc, then clamped the nail betwixt the ...clamps and powered up. By the way ( can't put btw it just feels wrong!!!!!) this was way before H&SE gone mad but he had a huge perspex shield for the class of grunty 17 year olds and he was wearing a mask

massive bang, nail literally exploded ( as Im sure you expected). Them were the days "ay it" still remember watching the chemistry teacher whimper and panic because he'd realised he had the phosphorus stick out too long and spent too long trying to scalpel a slice off, it lit up and he threw it down the sink in the fume cupboard! Water and phosphorus?!? Never did find out what happened to the drainage system