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What should you watch out for when undertaking building services upgrade works which require power from the building main switchboard?

Upgrades to mechanical services HVAC plants are common tasks which are typically either initiated by the age of the plant or by the drive to improve the building’s energy efficiency and NABERS rating.

One thing that often causes problems for building owners and project managers requesting this work is the inherent design of the building’s main switchboard (MSB) and the electrical mains cable from the MSB to the mechanical services switchboard (MSSB).

A building’s MSB is the parent switchboard for the premises. This switchboard will receive incoming electrical supply and distribute the power to a number of sub-switchboards that are responsible for light and power, lifts, mechanical services and the like.

Issues typically occur in buildings with MSBs dating back before the year 2000 as these will likely utilise a 1/3 size neutral cable feed to the sub switchboards. The older electrical standard allowed for electrical sub-main cabling to use a smaller neutral cable due to the fact that the neutral normally would not carry significant levels of current.

Under certain switching arrangements, with the use of VSDs and system harmonics, the neutral cable may be subject to significant currents. This has the potential to overheat the cable, creating a fire hazard.

The current electrical wiring standard AS3000-2007 requires that the neutral cable be sized to the same capacity as the active cables to ensure that the neutral cable can carry any unforeseen excess currents.

Issues often occur when a building owner provides a brief for the upgrade of HVAC or cooling systems. These brief may adequately identify the work as being limited to the mechanical services without requirement to change electrical power infrastructure.

For example, a brief may involve the replacement of a building’s chiller on the roof which is fed from an existing mechanical services switchboard. Part of this work would involve the re-use or upgrade (where required) of the mechanical services switchboard, but it would not necessary trigger an upgrade to the electrical sub-main supplying the MSSB as the total power demand would be the same or less (due to improvements in chiller efficiency).

The mechanical services work is briefed priced and awarded, equipment is ordered, works are underway and then the electrical sub-contractor undertaking the power supply connection works from the MSSB to the mechanical equipment discovers that the electrical sub-main to the MSSB has an undersized neutral supply. They are not able to provide certification of the work to current electrical safety standards unless the undersized neutral situation is rectified.

This scenario can create significant issues to the project including:

  • significant delays
  • increased unbudgeted cost (the MSSB may be on the roof of the building and MSB at the ground floor)
  • work requiring the shut down of the MSB and therefore the entire building.

In some cases, the MSB can be very old, and undertaking works within this switchboard can lead to discovery of other issues further increasing cost and time on the project.

In most cases, the workaround for the issue of the undersized neutral submain is to replace the older submain with a new submain or to provide parallel neutral cabling to supplement the existing neutral cabling capacity.

Another potential measure is to provide a means of electrical monitoring protection to the neutral cabling at the MSB. This ensures that any excessive current flowing to the neutral does not cause the clable to overheat. This may, however, be impractical due to lack of adequate space on the MSB to retrofit the neutral current monitoring device. This device monitors the neutral current and should the neutral conductor current carrying capacity be exceeded, it subsequently trips the MSSB supply’s circuit breakers.

This solution may not be practical, as it limits the MSSB supply capacity to the current carrying capacity of the neutral conductor (typically 1/3 the size). High neutral currents may occur when there are multiple single phase loads in operation as well as the presence of excessive harmonics in the electrical supply system.

All of the above mitigation measures will all require a shut down of the MSB, which will subsequently shut down the entire building power supply.

Those looking for mechanical services upgrades should allow for an electrical engineering review to determine whether:

  1. The submain to the switchboard being upgraded has adequate neutrla cable size to comply with current electrical safety standard.
  2. The MSB will accommodate works to connect additional neutral cable capacity or a suitable current protection device for the existing neutral cable (where it is found to be undersized).

A relatively small investment in cost and time will mitigate the risk of being caught out during the works by an existing latent condition related to the MSB submain neutral cable size.

With input from Jay Singh – senior electrical engineer at Meinhardt
 
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