Intrinsic Safety

[Jeff Stout]

Does RS-232 need a Zener Barrier, or can I send it out raw?

Jeff Stout

 

[Robert Baer]

Does RS-232 need a Zener Barrier, or can I send it out raw?

Jeff Stout

1) What the hell is a "Zener Barrier"?
2) The RS-232 spec allows a fault up to 25V difference between the
driver and the receiver, if i remember correctly.
However, in practice, there have been almost zero ICs that would work
anywhere near that offset.

 

[The Real Andy]

1) What the hell is a "Zener Barrier"?
2) The RS-232 spec allows a fault up to 25V difference between thear
driver and the receiver, if i remember correctly.
However, in practice, there have been almost zero ICs that would work
anywhere near that offset.

Now I am by far no expert when it comes to intrinsic safe devices, but I
thought that the requirement was for the powersupply to be intrinsically
safe. Please do not take my word on this, as I am just recycling some info I
heard in the last week when working with another company that supplies
intrinsically safe gear. Now the reason I believe this may be true, is
because I have sent gear off to be intrinsically approved and all they do is
pot the battery pack up with silicone and supply an intrinsically safe PSU..

I just signed up to do the stuff required to design intrinsic and explosion
proof gear, so hopefully I will know the real answer soon!!!

 

[Wim Ton]

I just signed up to do the stuff required to design intrinsic and explosion
proof gear, so hopefully I will know the real answer soon!!!

I worked for an oilfield serveice company where we used intrinsic safe
multimeters to check the continuity of the cables and firing caps. It was
basically a Fluke DMM with a resisor in the lead to the battery, so that
even in the case of a fault that would connect the battery directly to the
terminals, the current would be insufficien to set of the blasting cap.

Wim

 

[Tilmann Reh]

Now I am by far no expert when it comes to intrinsic safe devices, but I
thought that the requirement was for the powersupply to be intrinsically
safe. [...]

Intrinsic safety is not related to devices, but to electrical circuits.
It restricts the voltage, current, and stored energy in a circuit so
that generation of sparks is "impossible".

Anytime you connect a circuit that is not intrinsically safe to anything
within an "ex" area, you need those barriers to limit voltage, current,
and energy in the circuit that reaches into the ex area.

Back to the OPs question, I would not rely on RS-232 signals to
be intrinsically safe, especially if I didn't know how exactly the
driver circuit is built and/or protected. Particularly, you won't
get an Ex-i certificate for the device that has that RS-232 interface.

Adding a dual-port barrier gets you on the safe side.

--
Dipl.-Ing. Tilmann Reh
Autometer GmbH Siegen - Elektronik nach Maß.
http://www.autometer.de

==================================================================
In a world without walls and fences, who needs Windows and Gates ?
(Sun Microsystems)

 

[Mike Harrison]

Now I am by far no expert when it comes to intrinsic safe devices, but I
thought that the requirement was for the powersupply to be intrinsically
safe. [...]

Intrinsic safety is not related to devices, but to electrical circuits.
It restricts the voltage, current, and stored energy in a circuit so
that generation of sparks is "impossible".

Anytime you connect a circuit that is not intrinsically safe to anything
within an "ex" area, you need those barriers to limit voltage, current,
and energy in the circuit that reaches into the ex area.

Back to the OPs question, I would not rely on RS-232 signals to
be intrinsically safe, especially if I didn't know how exactly the
driver circuit is built and/or protected. Particularly, you won't
get an Ex-i certificate for the device that has that RS-232 interface.

As long as the outputs are suitably current-limited, and the inputs are clamped to a defined voltage
and suitably current-limited to protect the clamp, and control the power than can be fed into the
circuit from the pins, An IS RS232 port to T4 IIC would be entirely possible.
Generating the +/-12 v supplies in an IS way would be non-trivial but certainly doable, and if you
could live with RS423 (+/-5v) levels it would be even easier.

Of course you could not rely on a non-approved device's RS232 port to be safe, but if located
outside the hazard zone, it could be clamped and protected at the zone boundary and still function.

 

[nospam]

Does RS-232 need a Zener Barrier, or can I send it out raw?

Everything needs a zener barrier that it is the whole point. You rely on a
barrier to protect the area from electrical energy which could cause
ignition and you rely on the barrier to fail safe.

You do this so you don't have to rely on anything outside the safe area.

Inside the safe area there are rules about how much energy can be stored,
like no big capacitors etc.

I would be somewhat worried about what the RS232 line is connected to in
the safe area.

 

[Jeff Stout]

1) What the hell is a "Zener Barrier"?

http://www.am.pepperl-fuchs.com/products/productsubfamily.asp?division=24&productsubfamily_id=53

Jeff Stout

 

[Jeff Stout]

Back to the OPs question, I would not rely on RS-232 signals to
be intrinsically safe, especially if I didn't know how exactly the
driver circuit is built and/or protected. Particularly, you won't
get an Ex-i certificate for the device that has that RS-232 interface.

But any chip that calls itself an "RS-232" driver chip must have a certain
impediance and limits in voltage and current. I mean, when was the last
time
you saw a RS-232 wire generate a spark (which is the key question)?

But we are not talking common sense here. We're talking about huge
Bureaucracies (CSA, UL) which have rules.

Adding a dual-port barrier gets you on the safe side.

--
Dipl.-Ing. Tilmann Reh
Autometer GmbH Siegen - Elektronik nach Maß.
http://www.autometer.de

==================================================================
In a world without walls and fences, who needs Windows and Gates ?
(Sun Microsystems)

Jeff Stout

 

[nospam]

But any chip that calls itself an "RS-232" driver chip must have a certain
impediance and limits in voltage and current. I mean, when was the last
time
you saw a RS-232 wire generate a spark (which is the key question)?

By definition any signal which can be passed through a zenner barrier is
incapable of generating a spark. There are hundreds of thousands of
barriers out there passing signals incapable of generating a spark. What a
waste.

Perhaps we should do away with fuses as well. There must be millions of
fuses out there which will be scrapped before they blow.

But we are not talking common sense here. We're talking about huge
Bureaucracies (CSA, UL) which have rules.

When I had to provide controls for some equipment in a hazardous area I
found the requirements of intrinsic safety to be refreshingly based on
common sense.

 

[N. Thornton]

But any chip that calls itself an "RS-232" driver chip must have a certain
impediance and limits in voltage and current. I mean, when was the last
time
you saw a RS-232 wire generate a spark (which is the key question)?

I dont think it is. You cant rely on a silicon IC to current and
energy limit, as they fail to do so sometimes. There is a difference
between safe and intrinsically safe, and theres a difference between
safe when working as per spec, and genuinely safe.


Regards, NT

 

[Paul Burke]

Everything needs a zener barrier that it is the whole point. You rely on a
barrier to protect the area from electrical energy which could cause
ignition and you rely on the barrier to fail safe.

More to the point, Zener barriers are cheap and engineering time is very
expensive. You COULD make a case that you had fully assessed the
situation, documented all that and then decided that no barrier was
needed- then imagine standing up in court and defending that decision
after the plant blew up. £50 UK from RS- don't even bother thinking
about it.

Paul Burke