Laying out a control panel that can be worked on

A panel is built once and worked on for fifteen years, usually by somebody who was not there when it was built, often at night with the line stopped.

That is the design brief, and it is not the same brief as fitting the components in.

Segregate by signal type

The single most consequential layout decision, and it has to be made before anything is mounted.

Four categories, in separate trunking runs.

• Mains and motor. Highest energy, largest noise source.
• 24 volt control. Digital I/O, relays, contactor coils.
• Analogue and instrumentation. Screened, and the most vulnerable.
• Network and encoder. Screened, and vulnerable in a different way.

Where runs must cross, cross at right angles. Parallel runs couple; perpendicular ones largely do not.

Drive output cable is in a category of its own. It should be the shortest possible route from the drive to a gland plate, screened, and nowhere near anything else.

Heat rises, so plan for it

Panel thermal design is usually done by feel, and feel is optimistic.

Heat producers, drives, power supplies, transformers, go high. Heat sensitive items, and anything with a stated ambient limit, go low. A PLC directly above a drive is in the drive's exhaust.

Manufacturers state clearances above and below their equipment. They are not suggestions; they are the condition under which the derating curves apply.

Work out the heat load, add it up, and compare with what the enclosure can dissipate at the site's ambient. A panel in an unheated warehouse in February and the same panel in August are different thermal problems and the summer one wins.

Access

Every terminal should be reachable with a hand and a screwdriver, with the panel live and the door open, without moving anything else.

Terminals behind trunking, terminals under a mounting plate lip, terminals reachable only by removing an adjacent device: all of these are found during commissioning and lived with for fifteen years.

Fuses and isolators at the front and clearly identified. The person who needs them is under time pressure.

Leave room

Twenty percent spare DIN rail, twenty percent spare trunking capacity, and spare terminals in every group.

This is not generosity, it is a prediction. Every panel is modified. A panel with room is modified tidily; a panel without is modified by cable tying a relay to the side and running a wire across the door hinge.

Labelling that matches the drawing

Every wire numbered, every terminal numbered, every device tagged, and all of it identical to the schematic.

The commonest failure is a panel labelled to a convention nobody wrote down, or labelled to a drawing that was revised afterwards. Both mean the drawing cannot be used to trace a fault, which means the drawing is decoration.

Print the labels. Handwritten labels fade, and the ones that fade are always the ones needed.

Doors and moving parts

Anything on the door needs a cable loop that survives being opened ten thousand times. Use door hinge cable protectors, leave slack, and route the loop so it does not chafe.

Door mounted devices should be on the door only if they need to be seen or operated. Everything else belongs on the plate.

Earthing

One earth bar, star point, with everything returning to it. Mounting plates and doors earthed with a proper bonding strap, not relying on hinges.

Screens terminated at the gland plate, over the full circumference, at the point of entry. Not twisted into a pigtail and run to a terminal three hundred millimetres away, which converts the screen into an antenna and undoes most of its benefit.