Installation Safety Requirements

 

Safety Symbols

 

Various symbols are used on the instrument; they have the following meaning:

Caution, (refer to the accompanying documents)

 

 

 


Functional earth (ground) terminal!

 

 

The functional earth connection is required for safety purposes and to ground RFI filters.

 

 

Personnel

 

Installation must only be carried out by technically qualified personnel.

 

Enclosure of live parts

 

To prevent hands or metal tools from touching parts that may be electrically live (powered), the controller must be installed in an enclosure.

 

Caution: Live sensors


Do not connect live (powered) sensors to any signal input on the controller. Live sensors are sensors that must be connected to the main's supply. The controller has transient protection circuits connected between the inputs and the earth connection that might be damaged by live (powered) sensors.

 

Wiring

 

It is important to connect the controller in accordance with the wiring data given in this handbook. Take particular care not to connect AC supplies to the low voltage power supply input. Use copper wires for 24V DC power supply to the instrument. Ensure that the wiring of installations comply with all local wiring regulations. For example in the United Kingdom use the latest version of the IEE wiring regulations, (BS7671). In the USA use NEC Class 1 wiring methods.

 

Power Isolation

 

The installation must include a power isolating switch or circuit breaker. This device should be in close proximity to the controller, within easy reach of the operator and marked as the disconnecting device for the instrument.

 

Earth leakage current

 

Due to RFI Filtering there is an earth leakage current of less than 0.5mA. This may affect the design of an installation of multiple controllers protected by Residual Current Device, (RCD) or Ground Fault Detector, (GFD) type circuit breakers.

 

Over current 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 a 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 24V DC

 

  • relay or triac output to logic, DC or sensor connections;

  • any connection to ground.

 

The controller should not be wired to VAC.  The 24V DC power supply voltage across the connections and between the power supply and ground must not exceed 2.5kV. Where occasional voltage 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 set point set too high.

 

Where damage or injury is possible, we recommend fitting a separate over temperature protection unit with an independent temperature sensor, which will isolate the heating circuit. Please note that the alarm relays within the controller will not give protection under all failure conditions.

 

 

Installation requirements for EMC

 

To ensure compliance with the European EMC directive certain installation precautions are necessary as follows:

  • When using relay or triac outputs it may be necessary to fit a filter suitable for suppressing the emissions. The filter requirements will depend on the type of load. For typical applications we recommend Schaffner FN321 or FN612.

 

Routing of wires

 

To minimize the pick-up of electrical noise, the wiring for low voltage dc should be routed away from high-current power cables. Where it is impractical to do this, use shielded cables with the shield grounded at one end.