In the UK and Europe the standard LV system is 400/230V TN-S. The transformers needed to convert from the HV system (11kV in the UK) to LV should result in a standard LV system of this type. The designation TN-S describes the fact that the LV system neutral is separate from the earth system throughout the installation, with one neutral-earth connection at the transformer. There are various other configurations – TN-C-S, IT,TT etc but these are not suitable for data centres for various reasons, this is the subject of a separate blog.
The overriding concern for the designer is safety – life safety for personnel in the facility and the safety of the critical IT equipment. These are intimately linked. The danger to life of direct contact with the HV is prevented by insulation and secure containment and can be considered the easy part of the design. The difficult part is fault protection – ensuring that an earth fault on the HV system does not result in a voltage rise on the LV system that is dangerous to life or property.
The applicable safety standard is BS EN 50522:2010 “Earthing of Power Systems exceeding 1kV a.c”. Other information about the interface between the HV and LV systems is contained in BS 7040:2011+A1 2015 “Code of practice for the protective earthing of electrical installations”.
The permissible touch voltage is a function of time – nearly 800V for a duration of 10mS, falling to about 80V for durations of 10 seconds or more. Two basic methods exist for achieving this, separation of the HV and LV earth systems, and deliberately joining the two systems. Separation is difficult to achieve and is only used where site conditions mean that joining the two systems cannot be used.
Joining the two systems is only allowed where the resulting touch voltage under HV earth fault is less than the graph in figure 4 above. To achieve this a very low impedance between the combined earth system and the bulk earth must be achieved, meaning that an extensive system of LV earth electrodes is required. Historically a figure of 1 ohm was used, above this figure separation is required, below it the two earth systems could be joined. This is no longer recognised in BS 50522, and the designer must demonstrate by calculation, software modelling or the use of standard arrangements that safety has been achieved. The design process is illustrated in BS 50522 as a flow chart (below).