Maintenance bypass operation

Installation of a maintenance bypass removes the need to power down the load during routine maintenance or capacity upgrade of an Uninterruptible Power System ( UPS). The bypass switch has two positions:

Mains power and UPS power

Manual operation of the switch directs which source of power feeds the load. A maintenance bypass is normally wall mounted adjacent to the UPS.

Maintenance bypass switch types There are two types of maintenance bypass switch: Make-Before-Break (MBB) Break-Before-Make (BBM) It is normal to install a MBB switch in an installation which requires a 'break-free' power supply as the BBM typically suffers from a 5ms break when turned between switch positions.

    Internal or external bypasses Some UPS boast that they incorporate an internal maintenance bypass switch. This has two drawbacks
  • If the UPS needs to be removed the load will still need to be powered down
  • As the UPS cannot be totally switched off (dangerous ac and dc voltages may be present) the engineer cannot perform routine maintenance

'Plug and Play' or hardwired There are two types of bypass installation. An installation may be either hardwired or softwired (more commonly known as 'Plug And Play').

'Plug And Play' This uses standard IEC sockets and may be used with any UPS installation up to a maximum of 2kVA. Above this rating the hardwired approach is adopted due to the higher amperages involved. The 'Plug And Play' bypass helps to improve logistics co-ordination and reduce the higher installation costs associated with larger UPS and the more traditional hardwired approach. Its use of standard 'plug-in' connections removes the need to have an electrically qualified engineer attend site. An additional benefit is that UPS utilising the 'Plug And Play' approach are normally user commissionable.

Hardwired The standard IEC sockets used on a 'Plug And Play' bypass and UPS have a maximum supply current 10A. Their use is restricted to UPS up to 2k5VA for 240Vac installations and 1kVA for 110Vac installations. In such instances the bypass must be hardwired. A typical installation powers the UPS from a fused spur. The output from the UPS is hardwired through the bypass to the load.

For more details see WBS series