Triboflow Bag Leak Detectors or Cartridge Filters, Broken Bag Systems and Dust Monitors by Auburn Systems

Intrinsic Safety

By: Bob Newton
Vice-president
Product Development

Introduction Subject:
All Auburn TRIBO.series dust detectors are designed to operate safely in hazardous environments and comply with the following design guidance standards: EN 50-014 / EN 50-020, CSA STD C22.2 No. 142-M1987 & No. 157-92, FM Class 3600 & 3610.

Basic Design Concepts Employed:

· Use of an infallible power (mains) transformer to insure that no more than 30 vrms can appear in the low voltage circuitry of the instrument.

· Inclusion of a fused redundant Zener diode barrier to insure that an excessive voltage can not appear in the low voltage circuitry of the instrument due to the application of a foreign voltage at the 4-20mA output terminals.

· Where a relay output is installed, installation of a relay that provides adequate coil to contract spacing and isolation.

· Installation of an infallible current limiting resistor in series with the sensor connection terminal as an integral part of the encapsulated front-end module.

· Inclusion of a permanently connected Over Voltage Protector at each sensor probe to prevent excess voltage build-up on the probe, if the electronics should become disconnected.

· Maintain adequate creepage and clearance distances on circuit boards.

Infallible Transformers: The Model 2602 uses a Signal Transformer Co. Part Number 14A-10R-28 and the Model 2510 uses Part Number 14A-56-28 both of which are Type B construction with completely separate and consolidated primary and secondary winding, thus providing 4000 vrms primary to secondary isolation. A copy of the catalogue page for each is attached. These transformers also pass the 1.5 X fuse current rating temperature rise test required for qualification as infallible component.

Models 4001 and 4002 required a smaller transformer due to the enclosure size. It uses a modified version of Signal Transformer Co. Part Number 14A-5.0R-28. Although the VA rating of the standard transformer is adequate for the application, the smaller wire size used in the primary caused a higher than acceptable temperature rise during the 1.5 X fuse current rating temperature rise test. For this reason, Signal Transfer manufactures a custom version of the transformer, with a single primary winding incorporating an integral thermal fuse, for us with a special Part Number 14A-5.0R-1274 (for 115/120 v operation) or Part Number 14A-5.0R-1315 (for 230 v operation).

Barriers at 4-20mA Outputs: All output barriers use a redundant (parallel pair) of 16 volt 5 watt Zener diodes and a fast acting 1/16 A (63mA) 250-v fuse. This ensures that no more than +16 volts or -.06 volts can enter the low voltage circuitry, due to a foreign voltage at the output terminals. Each diode will individually withstand a continuous current of more than 3 times the fuse current rating, thus assuring that the 12T of the diode will always exceed that of the fuse. Also, an infallible current limiting resistor is included to insure that the current breaking capacity of the fuse cannot be exceeded.

Output Relay (Model 2602 and 4001): The output relay is an American Zettler Number AZ 2732-053-52 which provides 4000 vrms isolation between the coil and contacts. A copy of the catalogue data sheet for the relay is attached.

Infallible Current Limiting Resistor: An infallible resistor must be constructed such that no failure mode (usually burnout) cannot cause it to short circuit or become more conductive. An infallible resistor must also have a continuous power rating of at least 1.5 X the power that it would dissipate, should the sensor connection be shorted to the ground and a circuit fault cause line (mains) voltage to become present at the resistor.

A 150K ohm, 0.5w, MIL type RL20 resistor is included in the encapsulated front-end module for each sensor connection. The resistor meets the aforementioned requirements, protects the input circuit from voltage spikes and the encapsulation insures isolation of the sensor connection from comprise by other voltage sources.

Over Voltage Protector (OVP): During normal operation, the sensor probe is maintained very near ground (earth) potential (<0.1). If the ground (earth) return path for the sensor probe is broken by removal of the electronics from the housing or the disconnection of the coaxial cable, a static charge could accumulate on the probe. If a remote sensor is involved, the coaxial cable could become charged as well. Such a situation could be an ignition hazard, unless the maximum voltage is limited to a safe level. All sensor probes are provided with on or more gas discharge OVP’s that will conduct to ground (earth) and limit the probe voltage to ~ 90 v. Also, the special coaxial cable used for remote sensors is designed with a resistive center conductor thereby limiting the discharge current to a save level should the cable become damaged at any point in its length while charged to 90 volts.
Creepage and Clearance Distances: Generally, a minimum of 6mm clearance and 10mm creepage distances are maintained where required. However, in some cases, the 10mm creepage requirement becomes impossible to maintain on the circuit board. To resolve this problem, a grounded (earthed) and non-insulated guard conductor was interposed.