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intrimsically safe circuit, max volts, max amps on safe side.

intrimsically safe circuit, max volts, max amps on safe side.

intrimsically safe circuit, max volts, max amps on safe side.

(OP)
I would like to know the max allowed voltage and amperage on the safe side of an intrinsically safe circuit.
Thanks, EELV

RE: intrimsically safe circuit, max volts, max amps on safe side.

Hi EELV,

Your question had me going for a while.
It's the equipment that can be intrinsically safe,
the circuit can be what it likes.
The term 'intrinsically safe' is usually used in conjunction
with a description of the environment where the equipment is
intended for use. In some places spanners made of ferrous
metals (iron, steel etc) are not allowed because of sparks.

An 'intrinsically safe' piece of electrical equipment which
is physically separated from a hazardous environment by a
casing such as light switches with rubber covers actually
has the 'safe' space on the inside, cos thats where sparks
can happen without reaching the hazard. The casing has to
be made so as to cope with any reasonable 'occurrance' on
the inside, without allowing the 'occurrance' to affect
the outside. Such could include arcing, sparking, getting
hot or anything that has to be prevented from reaching the
outside of the separation.

On my way back from the chip shop struck me that i may
have mis-understood your question. You may have been
asking if there is a point below which small electrical
appliances could be regarded as safe, even in an otherwise
hazardous area.
No there isnt. Even hand held torches have to qualify.
Some newer types of prosthetics wouldnt qualify.

In the case of hazard due to causes of possible ignition,
i think it was twenty feet outside the designated area
although this is usually extended to the next sensible
edge or line, such as a perimeter fence, or the side of
a pathway, where 'no smoking' signs would be displayed.
Although these days probably the whole of the site
would be a no smoking area. So i dunno what the sign
would say. There is sure to be a new-look euro-sign thats
just the job.

In some cases it isnt always clear what the hazard is, or
why some items are forbidden. Some hospital areas forbid
the use of mobile telephones where the hazard is not to
the user of the mobile telephone, but to hospital
equipment. Some engine rooms bar certain types of
footwear because of possible sparking on metal treads.

So to your question:

"....max allowed voltage and amperage on the
safe side of an intrinsically safe circuit?"

If we substitute 'equipment' for 'circuit' then the
answer would be:

Within the manufacturers ratings should be ok.

because the safe side is inside, the hazard is outside.

A lot of thought went into this and if you meant something
else or if i read your question wrong, then come back and
say so. Come back anyway, cos i am most curious about it.

Regards, John

RE: intrimsically safe circuit, max volts, max amps on safe side.

Suggestion: There are other postings in this Forum pertaining to this subject, e.g. my posting:
Thread248-9036
Obviously, the smaller voltage, current and power than allowed by the codes/standards, the better safety is in effect.

RE: intrimsically safe circuit, max volts, max amps on safe side.

The rule of thumb I learned was about 1.0W. I'm not sure that there is an exact number, but it is derived from the heat needed to explode actylene gas ( the easiest to explode). See www.mtl-inst.com

RE: intrimsically safe circuit, max volts, max amps on safe side.

oh sh###t
  when we talk intrinsically safe circuits guys THERE AINT NO RULE OF THUMB>
The objective of an intrinsically safe circuit or device is that there is insufficient energy available in the circuit to ignite the hazard we are concerned about.
so we could have a very low voltage on a capacitor and short it. in a hydrogen atmosphere.
  There is some execelent literature by MTL and Pepper & Fusche on this subject. All components of the circuit in the hazardous zone must form part of the IS system. And they are selected relevant to the hazard to be expected.
  This really is an area where a lot of homework has to be done before you commit and take nobody's word for it - get a copy of the certification

Now I have got down from my soap box I do urge you to ge hold of some of the excellent publications the specialist suppliers have. And read up some of the texts.
John1 you have described explosion proof systems and I think you will find hazardous zones can extend much more than 20' in some applications.

As to values I can say I've had barriers shut down instruments at 24v dc and 30 mA when they have cycled the power. But the energy really is the issue

Regards Don

RE: intrimsically safe circuit, max volts, max amps on safe side.

Small encore: If you really would like to have a peace of mind, then it is better to try it with the hazardous material and environment present, and perhaps somewhat conservatively exaggerated.

RE: intrimsically safe circuit, max volts, max amps on safe side.

Hi Don01,
(from John1)
Yes you're right, I was on about various causes
of ignition, and the various precautions that
are often taken with this in mind.

And you are of course quite correct about the
energy content is the factor, not the voltage
or the current.

And you're right that this subject shouldnt be
treated lightly, and theres no general rule of
thumb, apart from 'get expert advice'  

And you're quite right, hazardous zones can
extend over very large areas, much greater
than 20 feet. i meant the warnings were usually
around twenty feet or so outside such areas,
typically by the next edge or line such as a
pathway or wall. These days they probably go
even further away from the extent of the
'hazardous area'

HOWEVER....
since Mr EELV doesnt look like he's gonna
re-appear its all rather irelevant, but i think
that he was talking about something else.

This bit about 'max allowed volts and amps'
That kind of niggled at me cos it rang a bell
faintly in the back of my head.

I reckon hes on about the low voltage systems
that are used on site-work.
There is somewhere a recommendation that they
shouldnt exceed 65 volts (i think) from ground
the arrangement that i have seen many times is
a 55-0-55 volt system where the centre tap is
earthed. This arrangement uses the familiar
110 volt equipment.

As to the current, i reckon hes on about the
trips. What should they be? Actually i dont
know but my guess would be that for a 55v pk
system 2mA would be acceptable, whereas house
mains probably less than 1mA would be required.

I think he actually meant to ask about that,
and if you read his question in that light,
it makes a lot more sense.

Well ... what do you think?

Regards,
John

RE: intrimsically safe circuit, max volts, max amps on safe side.

hi john
 yeh I hear you re: eelv's post and now you have me thinking.
Having reread my own post it does sound a bit strong don't it. It's just that I have a petrol-chem instrumentation background and have had a couple of very loud very close and very hot frights so its a subject that sets me off very easily.

I seem to recall the type of power system you describe in some instrument rooms around plants for calibrators and analysers. You see down here at the bottom of the world our standard is 240v and 110 is a "special" for some gear. Usually some thing like instruments or such imported from the states.
So an arrangement like you describe doesn't get of the ground for a 240v system and ours has a MEN system as the rule and it's damn hard to set up variations..

This was one reason the isolating and zener barrier type systems really hit he market big time for instrumentation.

You are aright it looks like eelv's gone off the air. I usually just enjoy talking to myself and the banter of my peers here. But it is good to get feedback from the guys who throw up the questions (usually pointing out the obvious thing I missed)

As a sidelight that ct system you describe would suit personal safety issues. My memory says 32mA is about the upper limit thru your body for a shock and voltages less than 65V ac are consider safe  for areas with a water hazard.

Regards Don

RE: intrimsically safe circuit, max volts, max amps on safe side.

Intrinsically safe, as defined in the Canadian Electrical Code, means that any spark or thermal effect that may occur in normal use, or under any conditions of fault likely to occur in practice, is incapable of causing an ignition of the prescribed flammable gas, vapour, or dust.

Intrinsically safe electrical equipment must be approved for the particular hazardous location where it is to be installed.  Don't know the maximum volts and amps, it probably varies with the type of gas, vapour, or dust, but if the equipment is approved for the location it becomes a moot point.

RE: intrimsically safe circuit, max volts, max amps on safe side.

hi redtrumpet
  what you say is correct and I hope thats what I said earlier.
As a rule the voltage and current etc are a moot point since you install a system of equipment. And don't forget the equipment is approved WITH each other. So you can't just grab a barrier and connect up any transmitter approved or not. The transmitter is approved with a specific range of barriers.

  I have had only one occassion only had to consider the matter of current and voltage on a loop.(in 20+ years of instruments) Bewarned those who use smart transmitters some of them do draw in excess of the barrier's limit when the do thier start up checking and communicating. It took Rosemount and MTL engineers near a whole day to figure out why the transmitter wouldn't get off the ground. Change the barrier to one to suit the smart tx and away we went.

Any way another prattle about that. All the best and catch you  soon
Don

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