Every once in a while, someone asks the question of whether fusing both sides of the power line is safe and permissible.
The first question that must be addressed is: What fault protection is the fuse providing?
There are two kinds of faults: (1) phase-to-neutral (pole-to-pole) and (2) phase-to-ground.
(Note that a fuse cannot provide protection for a neutral-to-ground fault because, by definition, the neutral is grounded. In a neutral-to-ground fault, the neutral and ground conductors become parallel conductors. In accordance with Kirchoff’s Laws, the current in the neutral goes down, not up. Hence, the over-current does not occur, and a fuse cannot provide protection).
A single fuse in the phase conductor provides protection for both kinds of faults.
A single fuse in the neutral conductor provides protection for neutral-to-phase faults, but not for phase-to-ground faults. This is one reason why a single fuse in the neutral is not permitted.
The second question that must be addressed is: Under what conditions does double-fusing provide the same or better protection than a single fuse?
We have already determined that a single fuse in the phase conductor provides adequate protection against both kinds of faults, and that a fuse in the neutral conductor does not. If double fusing is employed, the equipment is protected against both faults, but the neutral fuse is redundant for phase-to-neutral faults, and inoperative for phase-to-ground faults.
The only condition where fusing both phase and neutral conductors yields non-redundant protection against both faults is where polarity reversal is possible. That is, where the phase and neutral conductors could be interchanged on the supply side of the fuse. If polarity reversal is possible, then double-fusing guarantees that the phase conductor will always be provided with a fuse.
With double fusing, protection against both faults is provided for both normal polarity and reverse polarity.
The third question that must be addressed is: Is polarity (phase-neutral) reversal possible in the circuit on the supply side of the fuse? That is, is the fuse location (ie: phase or neutral conductor) constant or variable?
Building Wiring and Permanently-Connected Equipment
If we are dealing with building wiring or permanently-connected equipment, then fuse location is not variable, and polarity reversal is not possible. In this case, one fuse, in the phase conductor, provides protection for both phase-to-neutral and phase-to-ground faults.
The NEC, CEC, IEE Wiring Regulations and IEC 364 specifically prohibit fusing the neutral in building wiring and permanently-connected equipment.
If we are dealing with plug-and-socket-connected equipment, then we must examine the supply configuration, socket configuration, plug configuration, and wiring codes to determine whether fuse location is variable or not.
Three-Phase and Multi-Voltage Equipment
For three-phase (e.g. 208/120) and multi-voltage (e.g. 120-0-120) supplies, the plug and socket must maintain polarity in order to have functionality. In these cases, the fuse location is not variable because any polarity reversal (other than phase rotation) results in incorrect voltages applied to the equipment, usually with immediate catastrophic results, and opening of the building fuse or circuit breaker.
For these cases (plug-and-socket-connected three phase equipment and multi-voltage, e.g. 120-0-120 equipment), a fuse in each phase conductor provides protection for both phase-to-neutral and phase-to-ground faults. A fuse in the neutral conductor would be redundant and should it operate (open), the voltages applied to the various circuits will change and could cause overvoltage, overcurrent, and overheating conditions in at least one of the individual loads. For this reason, a fuse in the neutral must be prohibited, or it must be “ganged” with the phase conductor fuses such that if any one, including the neutral, operates, they all open.
Single Phase Equipment
For single-phase plug-and-socket-connected equipment, the plug and socket may or may not reliably maintain polarity, depending on the electrical code and the socket configuration.
Supposedly, the NEMA 5-l5R socket maintains the polarity of the building wiring, with the wide blade being the neutral conductor. However, there are several versions of the NEMA 5-l5P plug, some with wide blade and some without. Therefore, some plugs allow polarity reversal, while others do not.
In continental Europe, the socket wiring for the common 220 V, 16A plug is not polarized, and the equipment fuse location would be variable. In the UK and Australia, sockets and plugs are polarized, and the equipment fuse location would be constant.
The point is that each plug, socket, and building wiring is an independent situation which must be separately evaluated as to whether polarity reversal is possible. This, in turn, would make the equipment fuse location constant or variable.
The General Case for Single-Phase Plug-and-Socket-Connected Equipment
For single-phase, single-voltage plug-and-socket-connected equipment, single fusing ONLY provides protection for both faults when polarity reversal is not possible. If polarity reversal is possible, then a single fuse can only provide protection against phase-to-ground faults 50% of the time.
For single-phase, single-voltage plug-and-socket-connected equipment, double fusing ALWAYS provides protection for both kinds of faults regardless of whether polarity reversal is possible or not.
However, there are two hitches to double fusing.
First, when operating on a polarized system, some safety authorities insist that fusing be provided only in the phase conductor such that all of the equipment is de-energized for protection of the serviceman. This seems to require one fuse only.
However, this can be accommodated by using two, different value fuses. Select the fuse for the phase conductor (when connected to a polarized system) for proper overcurrent protection. Select the fuse for the neutral conductor to be one size larger than the phase conductor fuse. Thus, when connected to a polarized system, the smaller fuse properly opens for phase-to-neutral and for phase-to-ground faults. When connected to a non-polarized system and with reverse polarization, the smaller fuse provides protection for phase-to-neutral faults, and the larger fuse provides protection for phase-to-ground faults.
Second, some safety authorities insist that fusing be provided only in the phase conductor as required for building wiring. Any fuse in the neutral is cause for non-compliance of the equipment. The only solution here is to change our building codes and regulations to exempt single-phase plug-and-socket-connected equipment.