Scope:
This technical bulletin concerns the issue of neutral switching using Automatic Transfer Switches (ATS).
Summary:
Across the major ATS suppliers, there are 2 methods used to switch the neutral pole. The first method is via a fully rated 4 th /neutral pole assembly. The second is an overlapping neutral assembly. There is controversy in the industry as to which method is best. This bulletin gives a summary of each method, and GE’s position with respect to this issue. See GE Bulletin TB-1104 for a further discussion on 3-Pole vs. 4-Pole ATS applications.
Competing methods:
Method 1: Neutral switching via fully rated Neutral/4 th Pole
All GE Zenith 4-pole ATS utilize a fully rated, 4 th /neutral pole assembly. In this design, the neutral and phase poles are constructed on a common shaft and driven by a common mechanism. The rating of the neutral pole’s contacts is identical to the other phase pole contacts. Driving all four poles with a common shaft, assures that the Neutral pole will always transfer when the phase contacts transfer. In this way, the neutral contact cannot remain connected to the original source, nor can the load remain ungrounded due to failures of a dedicated neutral mechanism. The timing of the Neutral contact is adjusted so that it breaks its connection after the other phase contacts have opened and it closes into the new source before the other phase contacts have closed. When the Neutral is broken, the other phases have already been broken a few milliseconds earlier and therefore no current will flow. When the switch transfers, the Neutral will make first, and then the other phases will connect. In this way, a neutral connection is made before there is any power applied to the other phases.
Switching the sources in this manner minimizes switching transients.
Method 2: Neutral Switching via overlapping Neutral
The other method of neutral switching incorporates an overlapping neutral. In this case, the neutral in constructed in a make-before-break configuration. In most cases, the neutral pole is constructed differently than the phase poles (it has no arc chutes or arcing contacts) and it’s ampere rating is often less than the rating of phase poles. For these reasons, this construction is a far less expensive construction than supplying a fully rated neutral pole. The overlap neutral also operates on a mechanism that is tied to, but separate from, the main ATS operating mechanism.
GE Controls

GE’s Position:
GE rejects the overlap neutral design. UL1008, the governing standard for ATS in USA, also does not permit the under-rated overlap neutral ATS to be applied where switching of the neutral between the utility supply and generator supplies is required. In GE’s opinion, there are real dangers associated with the overlapping neutral design. Since the overlap neutral operates on a separate mechanism, there is potential that it can fail and not switch along with the phase poles, leaving both neutrals connected. A bigger danger is that the overlap mechanism will fail and leave both neutrals open, leaving the load ungrounded.
Claims have been made that damaging transients can occur as a result of the fact that the neutral is left ungrounded during the 30-50ms time period of the transfer. GE rejects these claims and is willing to evaluate any empirical or test data that suggests that such a damaging transient is possible. In theory, the only condition where a transient of significant magnitude can occur would be the case where the load connected to the ATS has significant regenerative/Back EMF capability (such as the case in a large motor). In this case, all manufacturers agree that a delay-type transfer switch with an intentional center-off position should be used. In this case, the reason why a delayed type switch should be used is not related to the neutral contacts, but is related to the voltage level mismatches between the phase contacts of the ATS upon reconnection of the regenerative load to the utility supply. In this case, all major manufacturers agree that switching large motors using an open transition switch is a misapplication of this type of ATS. If the load is not a regenerative load, there simply are not voltage mismatches sufficient to cause damaging transients.
Summary:
• If the neutral pole is to be switched, the safest and most reliable method is via a true
4 th /neutral pole assembly, with contact ratings equal to the phase poles, operated from the same mechanism and on the same shaft as the phase poles.
• The 4 th /neutral pole should be constructed for break-last and make-first to minimize any switching transients associated with switching of the neutral pole.
• Damaging transients associated with break-last/make-first neutral pole constructions are unsubstantiated.
• If large regenerative/back EMF loads are to be switched by the ATS, a delayed or full closed transition-type ATS should be considered to minimize transients and damage that can occur due to out-of-phase switching across the phase contacts.
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TB-1105A Page 2 of 2 5/16/2007