We have noticed that a few consultants are specifying ATS’s with overlapping neutrals. We believe this is because in the old days Auto Transfer Switches were only three pole devices or four pole devices with neutral poles that used to take longer to close than the phase poles. This could cause problems with ground current circulation and high transient voltages. So, manufactures used to get over this problem by using overlapping neutral poles.
The Mertech Automatic Transfer Switch is designed for use on separately derived systems (ie both supplies are grounded at source) as these are by far the most common in our experience. It is our recommendation that three pole devices or four pole devices with overlapping neutrals should not be used on this type of system and a modern four pole change over device should be used.
Mertech only use modern day switching devices that have the neutral pole on the same moving contact bar ensuring avoidance of surges.
It has been proven that the magnitude of transient voltages generated across the neutral switch of a modern 4-pole switch do not exceed the normal neutral-to-ground voltages ordinarily present on the neutral bus, even under worst case conditions of unbalanced load switching.
Therefore we suggest that 4-pole modern transfer switches should be used on any separately derived system as they successfully inhibit circulating ground currents that would otherwise attempt to flow between sources. Even short time overlapping neutrals can result in nuisance tripping of GF protection relays. Ground current cannot circulate through the neutrals of de-energised sources when 4- pole switches are used.
Mertech PC Automatic Transfer Switches utilise a Socomec motorised switching device which includes synchronised neutral opening & closing:
All contacts, including the neutral, are fitted on the same moving contact bar which ensures neutral referencing & avoids surges. This is SOCOMEC’s solution to the overlapping neutral “requirement”