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Guide to commercial installations | Part 2 Panelboard: Standards and Regulations
9
Specifiers must follow the requirements for switching neutral; these are shown in the table below:
Isolation requirements of the neutral conductor
Main switch at origin of installation
Outgoing device at the origin and devices
within the installation
Use by Ordinary persons
Use by Skilled or
Instructed persons
Use by Ordinary persons
Use by Skilled or
Instructed persons
System
Single
Phase
Three
Phase
Single
Phase
Three
Phase
Single
Phase
Three
Phase
Single
Phase
Three
Phase
TN-S
TN-C S
YES
NO*
NO*
NO*
NO*
NO*
NO*
NO*
TT
YES
YES
YES
YES
YES
YES
YES
YES
Notes
* Provision shall be made for disconnecting the neutral conductor e.g. a suitable terminal or bolted link
BS 7671 also requires that the device used for isolation is designed and/or installed to prevent unintentional or inadvertent
closure. You can normally achieve this by fitting a locking mechanism to the device.
IEE Guidance Note 2 provides more detailed guidance on isolation and switching.