Intrinsically safe phone circuit

[busman1]

Hi There everyone,
I need help in obtaining a circuit diagram for an intrinsically safe mine phone to be used in the mining industry in South Africa. The Problem: I have manufactured a underground phone for this purpose and it got sent to the Bureu of Standards here in S.A. it was certified I.S. Now it is not I.S. anymore because the control voltage is 48V and it needs to be 12V according to our 'standards experts' can anyone help me please.... i have invested alot of money already into the mould and manufacture and do not want to lose this money.CAN SOMEONE PLEASE ADVISE ME ON WHAT OPTIONS I HAVE, thank you

 

[Lewish]

If the phone enclosure is airtight, thus explosion proof, I would think you could still use 48volts. I do happen to know that the phones used in the coal mines in West Virginia are all 12volts. Or at least all the ones I have seen. Maybe the "standards" people are just reacting to what is done elsewhere rather than thinking for themselves. Sort of the "If everybody else is doing this, then it must be safe, so we will too" attitude.

 

[dpc]

The idea of intrinsically-safe equipment is that the available energy is low enough that ignition can not take place.

You might be able to satisfy everyone by introducing a very small dc to dc isolator rated as a barrier (I am assuming you are talking about 48v dc? - if ac you can use a transformer)that will inherently limit the current output.

For Lewish - explosion-proof enclosures are not really air-tight, as far as I know. They are vented in such a way that an explosion inside the enclosure will not produce exhaust gases that will ignite the surrounding atmosphere. This generally requires a larged machined surface that will cool any exiting gas down to a safe temperature.

 

[Lewish]

Hi dpc. I know that it is true that many IS enclosures are not sealed. However, the radios that I worked on the design of, were sealed and were filled with nitrogen.
I just thought that by sealing his enclosure, he could avoid some of the problems with the certifing group.

 

[nbucska]

Depending on the flammable gas, you may prefer to use
a metal mesh to allow the air pressure to equalize.
This would allow an explosion to happen inside of the
enclosure but would prevent the explosion to propagate
to the outside . It can even be done for H2-O2 mix !!
( See: Davy-lamps used in mines) <nbucska@pcperipherals.com>

 

[peebee]

Intrinsically safe phones are typically MUCH cheaper, lighter, and smaller than explosion-proof phones; you probably want to keep going down the IS path rather than exploring XP.

Intrinsically safe implies that the power produced by any spark is less than that required to ignite the atmosphere. That power may vary depending on the atomosphere in question. Both current limiting and voltage limiting are usually required to ensure power limiting. Energy-storing devices (inductors and capacitors) must also be sized to ensure that stored energy can non produce a spark of sufficient energy to ignite the atmosphere.

Voltage limiting is usually accomplished using zener diodes. Current limiting is usually accomplished using fuses. The power limiting equipment will likely need to be installed outside the XP area or provided with an XP enclosure.

The whole system should be listed by UL or other recognized testing firm.