24VDC grounding
- Thread starter lquadros
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Re: 24VDC grounding
I can help you out with this one. For one there is no requirement to ground a 24-volt system, however I would not suggest it nor have I seen anyone do it.
The purpose is the same as for any system, to provide an inexpensive method of conductor/circuit protection. Otherwise you would have to use either fuses or circuit breakers on each polarity with expensive ground fault detection and protective relays. It is just a lot easier and less expensive to reference one of the polarities (usually the negative return bus on 24 VDC systems, and positive on 48 VDC) to ground, install OCPD on the ungrounded conductor, and use a equipment frame ground to provide a fault clearing path back to the source (return bus). Same as AC systems, just different terms.
If you left it floating and one polarity were inadvertently grounded, nothing would happen, well maybe nothing, depends on how the end equipment is designed. But all that should happen is a ground fault detection alarm should sound and the equipment keeps churning along happy as can be.
I can help you out with this one. For one there is no requirement to ground a 24-volt system, however I would not suggest it nor have I seen anyone do it.
The purpose is the same as for any system, to provide an inexpensive method of conductor/circuit protection. Otherwise you would have to use either fuses or circuit breakers on each polarity with expensive ground fault detection and protective relays. It is just a lot easier and less expensive to reference one of the polarities (usually the negative return bus on 24 VDC systems, and positive on 48 VDC) to ground, install OCPD on the ungrounded conductor, and use a equipment frame ground to provide a fault clearing path back to the source (return bus). Same as AC systems, just different terms.
If you left it floating and one polarity were inadvertently grounded, nothing would happen, well maybe nothing, depends on how the end equipment is designed. But all that should happen is a ground fault detection alarm should sound and the equipment keeps churning along happy as can be.
petersonra
Senior Member
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- Northern illinois
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- engineer
Re: 24VDC grounding
From NFPA79
Note the requirement for ground fault indictaion or trip for ungrounded circuits.
8.3 Control Circuits. Control circuits shall be permitted to be grounded or ungrounded. Where grounding is provided, that side of the circuit common to the coils shall be grounded at
the control transformer if alternating current or at the power supply terminal if direct current.
Exception No. 1: Exposed control circuits as permitted by Section 6.3 shall be grounded.
Exception No. 2: Overload relay contacts shall be permitted to be connected between the coil and the grounded conductor where the conductors
between such contacts and coils of magnetic evices do not extend beyond the control enclosure.
8.3.1 Ungrounded control circuits shall be provided with an insulation monitoring device that either indicates a ground (earth) fault or interrupts the circuit automatically after a
ground (earth) fault.
Exception: Class 2 low-voltage circuits in Article 725 of NFPA 70, National Electrical Code, shall not require insulation monitoring.
From NFPA79
Note the requirement for ground fault indictaion or trip for ungrounded circuits.
8.3 Control Circuits. Control circuits shall be permitted to be grounded or ungrounded. Where grounding is provided, that side of the circuit common to the coils shall be grounded at
the control transformer if alternating current or at the power supply terminal if direct current.
Exception No. 1: Exposed control circuits as permitted by Section 6.3 shall be grounded.
Exception No. 2: Overload relay contacts shall be permitted to be connected between the coil and the grounded conductor where the conductors
between such contacts and coils of magnetic evices do not extend beyond the control enclosure.
8.3.1 Ungrounded control circuits shall be provided with an insulation monitoring device that either indicates a ground (earth) fault or interrupts the circuit automatically after a
ground (earth) fault.
Exception: Class 2 low-voltage circuits in Article 725 of NFPA 70, National Electrical Code, shall not require insulation monitoring.
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