Marathon Monitors Inc.
Overcurrent protection
To protect the internal PCB tracking within the controller against excess currents, the AC power supply to the controller and power outputs must be wired through the fuse or circuit breaker specified in the technical specification.
Voltage rating
The maximum continuous voltage applied between any of the following terminals must not exceed 264Vac:
∙line or neutral to any other connection;
∙relay or triac output to logic, dc or sensor connections;
∙any connection to ground.
The controller should not be wired to a three phase supply with an unearthed star connection. Under fault conditions such a supply could rise above 264Vac with respect to ground and the product would not be safe.
Voltage transients across the power supply connections, and between the power supply and ground, must not exceed 2.5kV. Where occasional voltage transients over 2.5kV are expected or measured, the power installation to both the instrument supply and load circuits should include a transient limiting device.
These units will typically include gas discharge tubes and metal oxide varistors that limit and control voltage transients on the supply line due to lightning strikes or inductive load switching. Devices are available in a range of energy ratings and should be selected to suit conditions at the installation.
Conductive pollution
Electrically conductive pollution must be excluded from the cabinet in which the controller is mounted. For example, carbon dust is a form of electrically conductive pollution. To secure a suitable atmosphere in conditions of conductive pollution, fit an air filter to the air intake of the cabinet. Where condensation is likely, for example at low temperatures, include a thermostatically controlled heater in the cabinet.
Over-temperature protection
When designing any control system it is essential to consider what will happen if any part of the system should fail. In temperature control applications the primary danger is that the heating will remain constantly on. Apart from spoiling the product, this could damage any process machinery being controlled, or even cause a fire.
Reasons why the heating might remain constantly on include:
∙the temperature sensor becoming detached from the process;
∙thermocouple wiring becoming a short circuit;
∙the controller failing with its heating output constantly on;
∙an external valve or contactor sticking in the heating condition;
∙the controller setpoint set too high.
AACC 2000 Carbon | 6 | Nov. 1, 1997 |
|
|
