Some time ago, Cundall was called in to investigate an incident at a government agency several months after they moved in to a refurbished office building.
One morning without warning, the office lighting started to flash, personal computers suddenly started to fizzle and then the screens went blank. Unclear as to exactly what was happening; managers sounded the fire alarm and evacuated the building. The fire brigade was called but there was no obvious fire damage.
The electrical contractor was called in and soon identified that the neutral had become disconnected on a three-phase distribution board feeding one of the office floors. The fault had destroyed over 100 personal computers and the lighting control system on the affected floor. Given the extent of the damage, Cundall was called in to examine the problem and help the agency with its insurance claim.
Initially the fault mechanism was not fully understood and the agency believed that the PCs and lighting controls had been damaged by a supply surge. The electrical supply company was asked to monitor the supply to the building for a month and whilst the supply was on the high side (240 Vac compared to a nominal 230 Vac), there was no evidence of any surge.
Merlin Gerin MGI1254 FP Switch Disconnector 125A
We sent the damaged 4-pole isolator, like the one illustrated above, away for forensic analysis and this showed that the neutral terminal had not been properly tightened by the installer, leading to arcing, overheating and eventually melting on the isolator with the neutral going open circuit.
In normal operation, the PCs and lighting controllers receive 230V ac between the phase and neutral conductors. If the neutral goes open circuit, it is hard to predict exactly what happens but the PCs and lighting controllers receive up to 400V ac. The situation is complicated by the switch-mode power units in the PCs but they are all eventually destroyed as one after another fails and the 400V ac appears across the remaining ones.
A similar but more serious incident occurred this year in a new council office building. A neutral link in a main distribution board failed, affecting four office floors and damaging over 900 PCs, displays, lighting controllers and BMS outstations.
Obviously, the integrity of the neutral is critical. All neutral connections on three-phase boards feeding single-phase loads need to be checked for tightness regularly and particularly in the first few months of operation.
It has become common practice to install 4-pole isolation where only 3-pole isolation is necessary. It is generally easier to ensure the integrity of the neutral link with 3-pole isolation. I would only recommend using 4-pole isolation in hospitals and similar medical installations where complete isolation is required for maintenance.
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