Wednesday, July 24, 2019

A Class I Division 2 is a location: (1) In which volatile flammable gases, flammable liquid-produced vapors, or combustible liquid-produced vapors are handled, processed, or used, but in which the liquids, vapors, or gases will normally be confined within closed containers or closed systems from which they can escape only in case of accidental rupture or breakdown of such containers or systems or in case of abnormal operation of equipment, or (2) In which ignitable concentrations of flammable gases, flammable liquid-produced vapors, or combustible liquid-produced vapors are normally prevented by positive mechanical ventilation, and which might become hazardous through failure or abnormal operation of the ventilating equipment.

Class I Division 2 Classification

Class I Division 2 refers to the ANSI/ISA 12.12.01 standard. This standard was previously UL1604 until UL recommended the newer ANSI/ISA standard be used and that all hazardous location products be certified under this standard by July 2012.

What is class 1 div 2

This standard applies only to equipment, circuits, and components designed specifically for use in Class I and II, Division 2 and Class III, Divisions 1 and 2 hazardous locations as defined by the National Electrical Code (NEC) ASI/NFPA 70.

Within the ANSI/ISA 12.12.01 standard. There are two types of hazardous conditions are 3 different classes and 2 divisions within each class. For a Class I rating the 2 divisions are Division 1 - normal conditions and Division 2 - abnormal conditions.

The CSA hazardous location certification is C22.2 No. 213-M1987-Non-Incendive Electrical Equipment for Use in Class I Division 2 Hazardous Locations.

What is a hazardous location?

According to the NEC and the Canadian Electrical Code (CEC) a hazardous location is defined as areas "where fire or explosion hazards may exist due to flammable gasses or vapors, flammable liquids, combustible dust, or ignitable fibers or flyings." Specifically, Class I Division 2 refers to only flammable gasses or vapors.

Furthermore, hazardous locations are locations where electrical equipment could be installed and might present a condition which could become explosive if the right elements for an ignition would be present. Hazardous material is expected to be found within closed containers and/or systems and would be present only if an accidental rupture, breakage or an unusual faulty operation would occur, leading the situation to be abnormal, the Division 2 rating.

An example of a normal condition would be where hazardous material would be present during normal plant operations, which coincides with the Division 1 rating.

This hazardous material could ignite through arcs and sparks, high temperature, and electrical
equipment failure.

This means that any product with a Class I Division 2 rating must contain an explosion within, provide a way for the burning gases to escape after the flames are quenched through a flame path.

Tuesday, July 23, 2019

Follow the steps below to select the correct electromechanical (EMRs) or solid state relay (SSRs) for your applications.

1.    Step 1. Identify mounting type:

a.    Panel
b.    DIN Rail
c.    Plug-in
d.    PCB

2.    Step 2. Identify Required Load Voltage Rating & Type:

a.    AC or DC
b.    Determine maximum voltage to be switched

3.    Step 3. Identify Required Load Current Rating:

a.    Calculate Average Load current
b.    Calculate Surge (inrush) current

4.    Step 4. Identify required circuit/switching arrangement:

a.    Number of poles/circuits to be switched
b.    Normally Open, Normally Closed or
c.    Changover switching function

5.    Step 5. Identify Required Control Voltage & Type:

a.    AC or DC
b.    Control Voltage Range

6.    Step 6. Identify Load Type:

a.    Inductive (Random Turn-on SSR)
b.    Resistive (Zero Crossing SSR)

7.    Step 7. Identify Type of SSR Required:

a.    Standard
b.    Special Application:

i.    Phase Angle Control
ii.    Burst Fire Control
iii.    Soft/Start Capability

8.    Step 8. Use above parameters to identify a relay (electromechanical or solid state) matching maximum load voltage, maximum surge current, load type, control voltage, type & mounting requirements.

9.    Step 9. Determine the amount of heat to be dissipated & select suitable thermal management solution.

Electromechanical Relays (EMR) Pros (Advantages)

•    Lower initial cost compared to solid state relays.
•    Provides complete electrical isolation.
•    Tolerates high current & voltage transients.
•    Insensitive to electromagnetic interference (EMI) / radio frequency interference (RFI).
•    Higher open resistance (air gap).
•    Lower closed resistance.
•    Available with many poles/circuits (up to 8 or more).
•    Many different circuit configurations available.
•    Multiple packaging & feature options.
•    Most typical failure mode is open.

Electromechanical Relays (EMR) Cons (Disadvantages)

•    Higher control (coil) power consumption.
•    Contact arcing can cause pitting & eventual open/short failure.
•    Contacts can be affected by corrosion, oxidation or contamination.
•    Contact bounce possible due to shock & vibration.
•    Generates electromagnetic interference (EMI) / radio frequency interference (RFI).
•    Can be orientation sensitive.
•    Can be affected by external magnetic fields.
•    Subject to mechanical degradation over time (residual magnetism, armature flexing, spring stretching).
•    Potential higher overall cost over equipment life.
•    Can be noisy.

Solid State Relays (SSR) Pros (Advantages)

•    Substantially longer life compared to mechanical relays.
•    Low control power consumption.
•    Faster on/off cycling.
•    Allows very fine proportional output control.
•    No arcing (safer in hazardous environments).
•    No contact bounce.
•    Not orientation sensitive.
•    Switching not affected by shock & vibration.
•    Not affected by external magnetic fields.
•    Less electromechanical interference.
•    Silent operation.
•    Allows many functions in a single package.
•    Potentially lower overall cost over equipment life.

Solid State Relays (SSR) Cons (Disadvantages)

•    Higher initial cost compared to mechanical relays.
•    Generate more heat compared to mechanical relays.
•    Current rating may require derating based on ambient temperature.
•    Voltage or current transients can damage or affect operation.
•    Susceptible to electromagnetic interference (EMI) / radio frequency interference (RFI).
•    Lower off state resistance.
•    Higher on state resistance.
•    Most are single pole/circuit (some available with up to 4).
•    Normally closed/ON function available on a limited basis.
•    Changeover form/circuit not supported.
•    Most typical failure mode is shorted/closed.