Designing a Distribution Board Enclosure: Busbars, DIN Rails and Barriers

A distribution board is planned from the inside out. The device list sets the rail length, the rail sets the enclosure width, the busbar and the cable space set the depth, and the segregation requirements set the barriers. Drawing the enclosure first and the internals second is the origin of most of the rework in this product family.

The Inputs a DB Layout Needs

DIN Rail Layout and Device Spacing

• Convert the device list into rail length, and add the spare capacity the specification requires.
• Keep consistent rail positions so that a pre-assembled row can be dropped into any enclosure of the same family.
• Allow for the depth of the deepest device plus the cable that terminates on it.
• Leave space at the ends of the rail for end stops and for the conductors that leave the row.
• Keep the device zone clear of the entry zone, so cabling does not have to pass over the devices.
• Where a row is split, provide a defined break so that the two sections can be wired independently.

Busbar Mounting and Support

• Provide supports at defined positions, with the spacing and the thread size on the drawing.
• Allow for the bar's own thickness, insulation and the terminating conductors when setting the depth.
• Keep the bar clear of the door through its full swing.
• Provide for bar covers and shrouds where the specification requires finger protection.
• Where bars are joined between sections, provide the aperture and the space for the joint.
• Keep the bar away from the cable entry zone, so that incoming cables do not have to cross it.

Internal Barriers, Shrouds and Segregation

• Design barrier mounting features into the enclosure rather than adding them later.
• Keep barriers removable where service requires access behind them.
• Where an incoming section is separated from an outgoing section, provide the aperture for the conductors and the space to terminate them.
• Where finger protection is required, provide shrouds that can be fitted and inspected.
• Where a barrier has to be insulating, agree the material and its thickness, because it affects the internal space.

Cable Space, Glanding and Bending

• Work out the space for the conductors including the bending radius of the largest cable.
• Provide glanding space behind the entry, since the gland itself occupies depth.
• Give the neutral and earth conductors a route of their own, so they do not cross the device zone.
• Provide cable supports where the cable mass would otherwise load the terminations.
• Remember that cable space and device depth compete for the same interior, which is the usual reason a board is too shallow.

Neutral and Earth Bars

• Put the bars in a defined zone, with the fixing points shown on the drawing.
• Position them so that conductors can be terminated without crossing the devices.
• Allow for the number of ways required, including spares.
• Provide a main earthing terminal with the size and position shown.
• Keep the bars accessible for inspection without dismantling the board.

Protection, Access and Locking

• State the protection class and the location together.
• Provide a lock type that matches the site's policy, including a tool-operated lock where required.
• Provide internal doors or covers where access to one section has to be controlled.
• Keep fixings and hinges out of the gasket line so that the seal is not interrupted.
• Where the board is outdoors or in a damp space, expect the material and the entry detail to change, not just the gasket.

What to Put in the Brief

1. Device list with module widths and the spare capacity required. 2. Busbar arrangement and support positions. 3. Cable schedule with entry positions and sizes. 4. Segregation and finger-protection requirements. 5. Location, protection class and finish. 6. Neutral and earth bar requirements. 7. Whether the board will be extended or repeated.

Checks Before Sign-Off

• Confirm the rail length and positions against the layout drawing.
• Confirm the busbar supports and their spacing.
• Check the internal depth with the devices and the cable allowance.
• Verify the entry positions and sizes from the cable schedule.
• Check the door and barrier fit and the gasket line.
• Verify earth continuity to the door and the barriers.
• Confirm the bars and their terminals are accessible and correctly positioned.

Our Approach to Distribution Board Layout

• We design the enclosure around the layout drawing, and we check the internal depth against the devices, the busbar and the cable allowance.
• We dimension the rail, busbar, barrier and bar fixing features from a single datum, so that a repeated board assembles identically.
• We will say when the requested device list and cable schedule will not fit the proposed enclosure depth.
• We do not select the devices, design the circuits or certify the finished board; that is the electrical designer's scope.
• Every quotation states which operations are completed in our plant.

Send the layout, the cable schedule and the segregation requirements, and the enclosure can be designed around the board rather than the board being reworked to fit the enclosure.

Frequently Asked Questions