Possible Correction:

Thread Starter

Macktek

Joined Jan 18, 2022
14
On p, 2611 of "Lessons In Industrial Instrumentation" by Kuphaldt...
It says, "In normal operation, the 4-20 mA field instrument possesses insufficient terminal voltage and
insufficient loop current to pose any threat of hazardous atmosphere ignition. However, the normally
modest voltage and current values within a healthy 4-20 mA loop circuit are enough for that circuit
to be considered intrinsically safe."


It seems to me it should say, "are not enough for that circuit to be considered intrinsically safe."

(Apologies if this is the wrong forum to post this, there was a link in the Kuphaldt PDF that lead here to report).
 

Thread Starter

Macktek

Joined Jan 18, 2022
14
Correction: "Are enough for that circuit to be considered Not Intrinsically Safe".
(I would have edited the original post, But, I don't see an edit button)
 

Thread Starter

Macktek

Joined Jan 18, 2022
14
It seems the definition is "not capable of creating an explosion" (probably via sparking, igniting).

According to the book this is what it says:
"However, the standard for nonincendive devices or circuits does not guarantee
what will happen under abnormal conditions, such as an open- or short-circuit in the wiring. So, a
“nonincendive” circuit may very well pose an explosion hazard, whereas an “intrinsically safe” circuit
will not because the intrinsically safe circuit simply does not possess enough energy to trigger an
explosion under any electrical fault condition. As a result, nonincendive circuits are not approved
in Class I or Class II Division 1 locations whereas intrinsically safe circuits are approved for all
hazardous locations."
 

ericgibbs

Joined Jan 29, 2010
15,332
Hi Mac,
You should include all the author's text in your evaluation.

However, the normally modest voltage and current values within a healthy 4-20 mA loop circuit

are enough for that circuit to be considered intrinsically safe."
 

drjohsmith

Joined Dec 13, 2021
376
Ain't the English language grate ..
:)
Thats why lawyers get so much .

Personally

I'd say a 40mA loop at the 9 or so volts can be made to spark ,
But we're into legal definitions here ...
 

Yaakov

Joined Jan 27, 2019
5,476
If the author intended to say a 4-20ma current loop circuit can be considered inherently safe, the paragraph is poorly written. If the author intended to say the current loop is non-incendive but not intrinsically safe, the word “are” should be changed to “aren’t” and this seems like an easy error to miss in proofreading.

What comes before and after that paragraph are important. If it goes on to explain (or had just explained) why something can be non-incendive while not being intrinsically safe then with the correction it is understandable If it doesn’t have the information in proximity it is bound to be confusing to the reader, even with the correction, unless it is assumed the reader knows this distinction which would certainly seem strange based on the presentation in this paragraph.

My surmise is the word “are” was meant to be the word “aren’t” to show that non-incendive is a necessary but insufficient requirement for intrinsically safe rating.
 

Thread Starter

Macktek

Joined Jan 18, 2022
14
Here is the entire page: (minus pics) + some from the preceding page for context:

So, a “nonincendive” circuit may very well pose an explosion hazard, whereas an “intrinsically safe” circuit
will not because the intrinsically safe circuit simply does not possess enough energy to trigger an
explosion under any electrical fault condition. As a result, nonincendive circuits are not approved
in Class I or Class II Division 1 locations whereas intrinsically safe circuits are approved for all
hazardous locations.

Most modern 4 to 20 mA analog signal instruments may be used as part of intrinsically safe
circuits so long as they are connected to control equipment through suitable safety barrier interfaces,
the purpose of which is to limit the amount of voltage and current available at the field device to
low enough levels that an explosion-triggering spark is impossible even under fault conditions (e.g.
a short-circuit in the field instrument or wiring). A simple intrinsic safety barrier circuit made from
passive components is shown in the following diagram:



1642586585808.png
In normal operation, the 4-20 mA field instrument possesses insufficient terminal voltage and
insufficient loop current to pose any threat of hazardous atmosphere ignition. However, the normally
modest voltage and current values within a healthy 4-20 mA loop circuit are enough for that circuit
to be considered intrinsically safe. In order to be intrinsically safe, the circuit’s voltage and current
levels must be limited even in the event of device or wiring faults.
This is the purpose of the intrinsic
safety barrier circuit: to serve as a safeguard in the event of unforseen wiring and/or component
faults so that there is no possible way for enough voltage or current to develop to trigger an explosion.
If a short-circuit develops in the field instrument, the series resistance of the barrier circuit will
limit fault current to a value low enough not to pose a threat in the hazardous area. If something
fails in the receiving instrument to cause a much greater power supply voltage to develop at its
terminals, the zener diode inside the barrier will break down and provide a shunt path for fault
current that bypasses the field instrument (and may possibly blow the fuse in the barrier). Thus,
the intrinsic safety barrier circuit provides protection against overcurrent and overvoltage faults, so
that neither type of fault will result in enough electrical energy available at the field device to ignite
an explosive atmosphere.


I believe that he intends to convey that 4-20mA circuits may be nonincendive but not necessarily intrinsically safe, which is why he adds the additional protection as shown in the picture..

Probably the wording should be changed to:
However, While the normally modest voltage and current values within a healthy 4-20 mA loop circuit are enough for that circuit to be considered intrinsically safe. “nonincendive”, in order to be intrinsically safe, the circuit’s voltage and current levels must be limited even in the event of device or wiring faults.
 

Thread Starter

Macktek

Joined Jan 18, 2022
14
In context, no change more than "are" to "aren't" seems necessary.
Hmm, I think you have the gist of it but, the actual result isn't what you are wanting.
Here's how it would look:
However, the normally modest voltage and current values within a healthy 4-20 mA loop circuit aren't enough for that circuit
to be considered intrinsically safe.


If the voltages aren't enough then it would suggest that the voltage is NOT enough ... which by implication, would suggest that it was already intrinsically safe.

The problem with the original sentence in the text is that the voltages are more than enough to be dangerous, so they are NOT intrinsically safe, so they need added protection. The sentence doesn't properly convey that.
 

Yaakov

Joined Jan 27, 2019
5,476
I don't agree. In context we know the sentence is referring to a "healthy" circuit (as was pointed out by @ericgibbs) above. This is the key word. It's not referring to the power in any possible state of a current loop in the absence of the safety barrier required to make it intrinsically safe, rather to the fact that it might appear so if abnormal operation is not considered.

I believe that it would require ignoring context to read that sentence as if it refers to the intrinsic power of a current loop circuit instead of the potential for exceeding the limits required for intrinsic safety in an abnormal state. After all, it is all aimed at pointing out the need for the safety barrier.

[EDIT: balancing unbalanced brackets]
 
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Thread Starter

Macktek

Joined Jan 18, 2022
14
The whole point of the book entry is that the 4-20mA is NOT intrinsically safe.
It doesn't matter how healthy the circuit is... it's not healthy enough to be intrinsically safe, and therefore it needs additional safety circuitry to make it safe enough to reach the level considered "Intrinsically Safe".
The voltage and/or current is too much, so it should not be considered Intrinsically safe... not the other way around.
 

Yaakov

Joined Jan 27, 2019
5,476
I don't think your logic makes sense. A healthy 4-20ma current loop will not exceed the limits of an intrinsically safe circuit. So the author is saying, in effect, "don't be fooled by the fact that a properly operating circuit appear to be safe, that isn't enough to consider it intrinsically safe and so we must add additional circuitry to make those limits intrinsic to our device."

In other words it is entirely consonant with the thesis of the book. There isn't a degree of "healthy" involved in his statement. An intrinsically safe circuit isn't "healthier" than one that does not fit that definition. Instead, "healthy" means operating within the expected parameters without a fault condition, which is as "healthy" as it can get. In that state, the loop is safe but it is not intrinsically safe because it is not intrinsically the case the circuit cannot exceed the limits.
 

ericgibbs

Joined Jan 29, 2010
15,332
hi,
If you feel so strongly about this point, why don't you contact the Author of the document, so he can defend his statements,
Post here his response.

You comment:
The voltage and/or current is too much, so it should not be considered Intrinsically safe... not the other way around.

Which Standards Authority has defined a 20mA loop as NOT Intrinsically safe.?
E
 

Thread Starter

Macktek

Joined Jan 18, 2022
14
Hmm, the link to post bugs was this forum. My assumption was that this was the place to put those.
Is this no longer the correct location?
 

Thread Starter

Macktek

Joined Jan 18, 2022
14
Sure. Discussion is welcome. I assumed there would be some official person that would review the bug posts for the book, since it linked to this location.
 

MrChips

Joined Oct 2, 2009
25,918
Sure. Discussion is welcome. I assumed there would be some official person that would review the bug posts for the book, since it linked to this location.
This is the correct location to post typo errors etc.
There is no official gopher who follows up on making corrections. Eventually the info will filter down to the original author.

Meanwhile, all the respondents here are reputable and knowledgeable members of AAC. They will create a sound consensus on the matter.
 

Thread Starter

Macktek

Joined Jan 18, 2022
14
I don't think your logic makes sense. A healthy 4-20ma current loop will not exceed the limits of an intrinsically safe circuit. So the author is saying, in effect, "don't be fooled by the fact that a properly operating circuit appear to be safe, that isn't enough to consider it intrinsically safe and so we must add additional circuitry to make those limits intrinsic to our device."

In other words it is entirely consonant with the thesis of the book. There isn't a degree of "healthy" involved in his statement. An intrinsically safe circuit isn't "healthier" than one that does not fit that definition. Instead, "healthy" means operating within the expected parameters without a fault condition, which is as "healthy" as it can get. In that state, the loop is safe but it is not intrinsically safe because it is not intrinsically the case the circuit cannot exceed the limits.
I think what he is saying is that in an otherwise healthy 4-20mA circuit, you could have some other external factor which causes an unsafe situation that would render the circuit not intrinsically safe. So he is recommending the added protective circuits.
(perhaps here we are saying the same thing)?
 
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