Wiring for Hazardous Location Question?

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goodoboy

Senior Member
Location
Houston
Hello,

I have a question regarding the wiring practice for hazardous
locations. I am still learning.

Please see picture.

The RTD wired to an explosion proof terminal head rated for Class 1 Div 2, inside a Class 1 Div 2 Group
building. Wiring runs outside of the building (explosion proof building)
through a conduit seal (to prevent passage of gas or vapors) and terminates in a
junction box. From there the signal goes to the TT and off to the control room.
The TT is not explosion proof enclosure , just standard.

Is this safe wiring?

Should an intrinsically safe barrier be installed between TT or
JB? Or maybe replace TT with instrinsically safe, explosion proof?

I think the wiring is not safe cause high energy level can come from the TT or control room and transmitte the hazardous area and cause a spark through the wire.

Thanks for the help in advance.
 

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rbalex

Moderator
Staff member
Location
Mission Viejo, CA
Occupation
Professional Electrical Engineer
The primary concern I would have is the TE itself. Is it "factory sealed" or not? If it is, I see no problem in a Division 2 location. If it isn't, an "intrinsically safe" barrier wastes the explosionproof enclosure since it would no longer be necessary. In addition, you didn't mention (or I've failed to understand) the actual wiring method used to route the mV wiring. That is, is it in a recognized Division 2 wiring method [Section 501.10(B)]

If you aren't VERY familiar with it, "Intrinsically Safe Systems" is a can of worms you probably don't want to open. See Article 504, there's a lot of minutiae to deal with. [You can thank your ISA buddies for that]
 

goodoboy

Senior Member
Location
Houston
The primary concern I would have is the TE itself. Is it "factory sealed" or not? If it is, I see no problem in a Division 2 location. If it isn't, an "intrinsically safe" barrier wastes the explosionproof enclosure since it would no longer be necessary. In addition, you didn't mention (or I've failed to understand) the actual wiring method used to route the mV wiring. That is, is it in a recognized Division 2 wiring method [Section 501.10(B)]

If you aren't VERY familiar with it, "Intrinsically Safe Systems" is a can of worms you probably don't want to open. See Article 504, there's a lot of minutiae to deal with. [You can thank your ISA buddies for that]

Thanks,

What do you mean factory sealed? Its a dual RTD in a pipe (thermowell) which sites in liquid for temperature reading.

Why did you state: "I see no problem in a Division 2 location"

What do you mean "If it isn't, an "intrinsically safe" barrier wastes the explosionproof enclosure since it would no longer be necessary. "

Thanks for help
 

rbalex

Moderator
Staff member
Location
Mission Viejo, CA
Occupation
Professional Electrical Engineer
Thanks,

What do you mean factory sealed? Its a dual RTD in a pipe (thermowell) which sites in liquid for temperature reading.

Why did you state: "I see no problem in a Division 2 location"

What do you mean "If it isn't, an "intrinsically safe" barrier wastes the explosionproof enclosure since it would no longer be necessary. "

Thanks for help
Why is the TE in an explosionproof enclosure in Division 2 in the first place unless it encloses arcing, sparking or high temperature (ASH) components?

If
it encloses ASH components, then the TE must either be factory sealed or field sealed.

If it doesn't enclose ASH components, then the TE doesn't require an explosionproof enclosure in Division 2 and intrinsically safe wiring is a waste of money.

If you use intrinsically safe wiring then the TE doesn't need to be explosionproof in either Division 1 or 2.​

You still haven't clarified the actual wiring method used in the Division 2 location.
 

goodoboy

Senior Member
Location
Houston
Why is the TE in an explosionproof enclosure in Division 2 in the first place unless it encloses arcing, sparking or high temperature (ASH) components?

You still haven't clarified the actual wiring method used in the Division 2 location.

I need to confirm with the contractor of the vendor how they plan to wire the loop in inside the Class 1, Division 2 building. I did not know that Division 2 requires a wiring method. But I will read into this as you mentioned above.

Currently, I am trying to decide if IS is required or not.

You are correct, why is vendor ordering explosion proof for the TE, its only mV signal back to the TT in the non-hazardous area. The measured temperature not get too hot. I will ask vendor whats the reason for the explosion proof TE head.


Thanks for your help and comment
 

goodoboy

Senior Member
Location
Houston

If it doesn't enclose ASH components, then the TE doesn't require an explosionproof enclosure in Division 2 and intrinsically safe wiring is a waste of money.


Thanks for the help

I am still not understand why intrinsically safe wiring is not required.

The TE does not contain any ASH components.

What happens if over-current occurs from the TT or the control room? This high current will enter the hazardous location through the wiring correct, and this could ignite explosion? This is the reason I believe IS is needed
atleast between the TT and junction box, please correct me if my thinking is incorrect.

Can over-current from the Control Room or TT ignite in the Class 1 Div 2 area through the wiring?

Thank you, sorry if I sound confusing.

Thanks​
 

rbalex

Moderator
Staff member
Location
Mission Viejo, CA
Occupation
Professional Electrical Engineer
Yes, "what if?" That's what Division 2 is all about. Division 1 deals with both normal and abnormal sources of ignition; Division 2 only deals with "normal" sources since there isn't supposed to be any gasses to ignite in Division 2 under normal conditions.
 

goodoboy

Senior Member
Location
Houston
Yes, "what if?" That's what Division 2 is all about. Division 1 deals with both normal and abnormal sources of ignition; Division 2 only deals with "normal" sources since there isn't supposed to be any gasses to ignite in Division 2 under normal conditions.

I guess I just don't understand. I am lost in understanding.:?:?

I just don't get it. you can call me if its better to explain.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
...
Currently, I am trying to decide if IS is required or not. ...
IS is never required, it is an optional installation method. There are always other methods that are permitted to be used.

...You are correct, why is vendor ordering explosion proof for the TE, its only mV signal back to the TT in the non-hazardous area. The measured temperature not get too hot. I will ask vendor whats the reason for the explosion proof TE head. ...
Vendors will almost always provided explosion proof devices if you tell them that the equipment is to be used in a classified area. Not sure if that is because they don't understand what the code permits or if they just always provide the more expensive explosion proof equipment because them make more profit selling it.
 

goodoboy

Senior Member
Location
Houston
IS is never required, it is an optional installation method. There are always other methods that are permitted to be used.


Vendors will almost always provided explosion proof devices if you tell them that the equipment is to be used in a classified area. Not sure if that is because they don't understand what the code permits or if they just always provide the more expensive explosion proof equipment because them make more profit selling it.

Thanks for replying:

The TE is small mV voltage during normal operation, so

What happens if over-current occurs from the TT or the control room? This high current will enter the hazardous location through the wiring correct, and this could ignite explosion? This is the reason I believe IS is needed
atleast between the TT and junction box, please correct me if my thinking is incorrect.

Can over-current from the Control Room or TT ignite in the Class 1 Div 2 area through the wiring?
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Thanks for replying:

The TE is small mV voltage during normal operation, so

What happens if over-current occurs from the TT or the control room? This high current will enter the hazardous location through the wiring correct, and this could ignite explosion? This is the reason I believe IS is needed
atleast between the TT and junction box, please correct me if my thinking is incorrect.

Can over-current from the Control Room or TT ignite in the Class 1 Div 2 area through the wiring?

Please re-read what Bob said (my emphasis added):
Yes, "what if?" That's what Division 2 is all about. Division 1 deals with both normal and abnormal sources of ignition; Division 2 only deals with "normal" sources since there isn't supposed to be any gasses to ignite in Division 2 under normal conditions.

Your hypothetical over-current from TT is not a normal source of ignition, and so is not considered to be a problem that must be addressed in a Division 2 location.

If some installation required three abnormal situations (or failures) to cause an explosion, it would be even less relevant. (Unless, of course, one predictable underlying event could simultaneously cause all three abnormal states, but the code does not specifically address that.)
 

rbalex

Moderator
Staff member
Location
Mission Viejo, CA
Occupation
Professional Electrical Engineer
...
(Unless, of course, one predictable underlying event could simultaneously cause all three abnormal states, but the code does not specifically address that.)
Actually, the Code does address it if were reasonably predictable that all three would be simultaneous events - it would make it Division 1. [Section 500.5(B)(1)(3)]
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
...
What happens if over-current occurs from the TT or the control room? This high current will enter the hazardous location through the wiring correct, and this could ignite explosion? This is the reason I believe IS is needed
at least between the TT and junction box, please correct me if my thinking is incorrect. ...
You don't seem to understand the difference between a Division 1 location and a Division 2 location.
A Division 2 location does not have flammable concentrations of vapors under normally operating conditions. Since it does not have the flammable concentrations under normal conditions, it would take a simultaneous failure of both the process system and the electrical system to cause a fire or explosion. Such an occurrence would be rare, so the code has less stringent rules for Division 2 areas than it has for Division 1 areas.
 

rbalex

Moderator
Staff member
Location
Mission Viejo, CA
Occupation
Professional Electrical Engineer
I guess I just don't understand. I am lost in understanding.:?:?

I just don't get it. you can call me if its better to explain.
Think of this in terms of the fire triangle in its simplest form - you need oxygen, fuel and an ignition source. Theoretically, we can eliminate any one of them to prevent a fire or explosion.

  • Within the reasonable constraints of most industrial applications oxygen is always available.
  • In Division 1, fuel is also available under "normal" conditions. (Note "normal" does not necessarily mean common or usual.)
  • In Division 2, fuel is only available under abnormal conditions.

SO - we usually limit the need to deal with ignition sources:

  1. In Division 1 we deal with both common and abnormal ignition sources since fuel is available under normal conditions.
  2. In Division 2 since fuel is abnormal we only deal with common ignition sources.

However, there are several recognized protection techniques for dealing with either the availability of fuel or ignition sources. See Section 500.7 for a list and where they are considered appropriate. Note Intrinsic Safety is simply one of them; it is permitted but not required.
 

goodoboy

Senior Member
Location
Houston
Please re-read what Bob said (my emphasis added):


Your hypothetical over-current from TT is not a normal source of ignition, and so is not considered to be a problem that must be addressed in a Division 2 location.

If some installation required three abnormal situations (or failures) to cause an explosion, it would be even less relevant. (Unless, of course, one predictable underlying event could simultaneously cause all three abnormal states, but the code does not specifically address that.)

Thanks for responding

Its slowly sinking in after doing some reading. I am thankful for this board. I am still learning electrical systems and 3 times this years this forum has help me.

To increase my learning, I will make a new task to come to the forum at least 2-3 times a week and help someone else question.
 

goodoboy

Senior Member
Location
Houston
You don't seem to understand the difference between a Division 1 location and a Division 2 location.
A Division 2 location does not have flammable concentrations of vapors under normally operating conditions. Since it does not have the flammable concentrations under normal conditions, it would take a simultaneous failure of both the process system and the electrical system to cause a fire or explosion. Such an occurrence would be rare, so the code has less stringent rules for Division 2 areas than it has for Division 1 areas.

Thank you for responding.

Yes, its fairly new to me both concepts. I am new to electrical systems practice, but learning. Even when not at work.

I now understand that the current configuration in the drawing is within NEC code.

1. The TE does not require explosion due no electrical circuitry within the TE head.
2. Intrinsically safe protection is not required because under normally operating condition the location does not contain flammable concentrations.

Just for the sake of learning. I am thinking how this would change if the TT was located in the hazards location. Then the TT will have to be sealed due to ASH parts in the TT head. But no other protection is needed since Division 2 location and under normal operations there are no contain flammable concentrations. Also, I would make TT explosion proof just incase vapor leaks in the electrical components of the TT. But does the NEC code require such protection for the TT is worthy of my reading and understanding.
 
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