Ground Fault Detection and Solidly Connected Transfer Switches Designing electrical systems utilizing emergency stand-by systems, legally required standby systems or even your basic optional standby systems can be challenging. The National Electrical Code® provides us with the minimum safety requirements. However, there are situations where selecting the correct transfer switch can be challenging and making the wrong selection can be downright dangerous. We are going to look at the transfer switch selection process and determine if emergency systems should utilize a 3-pole or should the correct choice be to utilize a 4-pole automatic transfer switch.
2020 National Electrical Code, with a few exceptions, only allows for a neutral to ground connection to occur once in an electrical system per section 250.24 (A)(5). When utilizing a transfer switch with a solidly connected neutral between the normal source and the emergency source the neutral to ground bond that may be factory installed for most generator sets and must be physically removed. When operating in generator mode with this configuration the ground fault path to trip the overcurrent protection at the generator must run through the equipment grounding conductor and then back to the neutral to ground bond at the service (either at the first disconnecting means or upstream) and then through the neutral all the way back through the transfer switch to the overcurrent protection at the generator.
When working with Emergency Systems, the designer and/or installer must to be aware of section 700.6 (D) of the 2020 NEC. This section requires ground fault detection be provided on grounded wye system at 150 V to ground with overcurrent protection 1000 A or greater. Current flows through the current transformer of the Ground fault detection which encompasses the phase and neutral conductors. When a ground fault occurs some current will bypass the current transformer causing the ground fault detector to indicate a fault.
As a result of the aforementioned condition, when utilizing a solidly connected neutral transfer switch in generator mode the ground fault will travel back through the neutral and through the current transformer of the ground fault detection device. As the total returned current will be roughly equivalent to that in normal operating mode the ground fault detection will not provide indication upon a ground fault.
As a result of my analysis of solidly connected neutral transfer switches, it is my interpretation that utilizing a transfer switch with a solidly connected neutral would not be permitted in a system that requires ground fault protection as this installation violates the intent of section 700.6 (D).
In an effort to help develop a safer and more reliable electrical design intend to submit a public input for the 2023 National Electrical Code Development Process that will seek to add clarity to the proper installation intent of section 700.6 (D) to indicate that the Ground Fault Detection must operate in the event of a ground fault condition and to also submit a recommended informational note indicating that transfer switches with solidly connected neutrals may impede the proper operation of the ground fault detection.
Authored By : Kyle C. MacKenzie P.E