System earthing arrangements for installations comprising UPSs

Application of protection systems, stipulated by the standards, in installations comprising a UPS, requires a number of precautions for the following reasons:

• The UPS plays two roles
  • A load for the upstream system
  • A power source for downstream system
• When the battery is not installed in a cabinet, an insulation fault on the DC system can lead to the flow of a residual DC component

This component can disturb the operation of certain protection devices, notably RCDs used for the protection of persons.

Protection against direct contact

(see Fig. N23)

All installations satisfy the applicable requirements because the equipment is housed in cabinets providing a degree of protection IP 20. This is true even for the battery when it is housed in a cabinet.

When batteries are not installed in a cabinet, i.e. generally in a special room, the measures presented at the end of this chapter should be implemented.

Note: The TN system (version TN-S or TN-C) is the most commonly recommended system for the supply of computer systems.

Fig. N23 – Main characteristics of system earthing arrangements

Type of arrangement	IT system	TT system	TN system
Operation	Signaling of first insulation fault Locating and elimination of first fault Disconnection for second insulation fault	Disconnection for first insulation fault	Disconnection for first insulation fault
Techniques for protection of persons	Interconnection and earthing of conductive parts Surveillance of first fault using an insulation monitoring device (IMD) Second fault results in circuit interruption(circuit-breaker or fuse)	Earthing of conductive parts combined with use of RCDs First insulation fault results in interruption by detecting leakage currents	Interconnection and earthing of conductive parts and neutral imperative First insulation fault results in interruption by detecting overcurrents (circuit-breaker or fuse)
Advantages and disadvantages	Solution offering the best continuity of service (first fault is signalled) Requires competent surveillance personnel (location of first fault)	Easiest solution in terms of design and installation No insulation monitoring device (IMD) required However, each fault results in interruption of the concerned circuit	Low-cost solution in terms of installation Difficult design (calculation of loop impedances) Qualified operating personnel required Flow of high fault currents

Essential points to be checked for UPSs

Figure N24 shows all the essential points that must be interconnected as well as the devices to be installed (transformers, RCDs, etc.) to ensure installation conformity with safety standards

Fig. N24 – The essential points that must be connected in system earthing arrangements