For the safety of service personnel performing work on or near an open coil duct heater, contactors used to control them must break all ungrounded conductors.
Paragraph 24.3.18 Requirement: Mercury or magnetic contactors used on open coil electric duct heaters shall break all ungrounded conductors. Phase break on three phase heaters shall not be permitted. Where silicon controlled rectifiers (SCRs) are used, the safety contactor shall break all ungrounded conductors. This requirement does not apply to sheathed type electric duct heaters.
