tgifredday
Member
Is there a problem with installing 24VDC, 4-20 ma instrument, in a Class I, Div 2 area that does not utilize I.S. or Non-Incendive wiring? We are
planning on using rigid conduit and fittings and all wiring practices applicable in a class I, Div 2 area. We use a 250 Ohm resistor in series with our wiring to limit the current and prevent a ground or short from affecting our power supply. The code reference below allows the use of a
general purpose enclosure if the transmitter has no make-and -break or sliding contacts and that we don't exceed 80% of the ignition temperature. The instrumentation we buy is typically rated I.S. or Non-Incendive, but the circuit we will be using will not be an approved I.S.or Non-Incendive if we do not use appropriate barriers. The 250 ohm resistor will current limit our 24VDC to about 100 ma and the instrument itself is rated I.S. or Non-Incendive and should not be capable of storing enough energy to ignite an explosive atmosphere (almost all of our classified areas are Class I, Div 2, Group D)and it does not have make-and break or sliding contacts. It's hard to believe we struggle with approving an instrument installation at 24VDC with short circuit current of 100 ma when we have 480V motors up to 100 HP in our processes.
In summary, is there a problem with installing standard wiring (non I.S. or incendive) with an I.S. or non-incendive rated instrument in a Class I, Div 2, Group D area?
501.3(B)(2) Resistors and Similar Equipment. Resistors, resistance devices, thermionic tubes, rectifiers, and similar equipment that are used in or in connection with meters, instruments, and relays shall comply with 501.3(A).
Exception: General-purpose-type enclosures shall be permitted if such equipment is without make-and-break or sliding contacts [other than as
provided in 501.3(B)1)] and if the maximum operating temperature of any exposed surface will not exceed 80 percent of the ignition temperature in degrees Celsius of the gas or vapor involved or has been tested and found incapable of igniting the gas or vapor. This exception shall not apply to thermionic tubes.
planning on using rigid conduit and fittings and all wiring practices applicable in a class I, Div 2 area. We use a 250 Ohm resistor in series with our wiring to limit the current and prevent a ground or short from affecting our power supply. The code reference below allows the use of a
general purpose enclosure if the transmitter has no make-and -break or sliding contacts and that we don't exceed 80% of the ignition temperature. The instrumentation we buy is typically rated I.S. or Non-Incendive, but the circuit we will be using will not be an approved I.S.or Non-Incendive if we do not use appropriate barriers. The 250 ohm resistor will current limit our 24VDC to about 100 ma and the instrument itself is rated I.S. or Non-Incendive and should not be capable of storing enough energy to ignite an explosive atmosphere (almost all of our classified areas are Class I, Div 2, Group D)and it does not have make-and break or sliding contacts. It's hard to believe we struggle with approving an instrument installation at 24VDC with short circuit current of 100 ma when we have 480V motors up to 100 HP in our processes.
In summary, is there a problem with installing standard wiring (non I.S. or incendive) with an I.S. or non-incendive rated instrument in a Class I, Div 2, Group D area?
501.3(B)(2) Resistors and Similar Equipment. Resistors, resistance devices, thermionic tubes, rectifiers, and similar equipment that are used in or in connection with meters, instruments, and relays shall comply with 501.3(A).
Exception: General-purpose-type enclosures shall be permitted if such equipment is without make-and-break or sliding contacts [other than as
provided in 501.3(B)1)] and if the maximum operating temperature of any exposed surface will not exceed 80 percent of the ignition temperature in degrees Celsius of the gas or vapor involved or has been tested and found incapable of igniting the gas or vapor. This exception shall not apply to thermionic tubes.