In the previous three Parts, we shared the OSHA requirements when using portable generators in the workplace and how to determine or test to see if your generators are compliant.
Now, in Part 4 we’ll transition to the reasons why a lack of neutral bonding can contribute to electrical hazards and an increased risk to workers when using portable generators with floating neutrals.
Additionally, we will discuss other safety hazards and some mitigation tips, both electrically and non-electrically related, when using portable generators.
What is the Potential Harm to Electrical Workers?
There are two main reasons why proper neutral bonding is related to employee safety for protection against electric shock.
Compromise the GFCIs and OCPDs
First, a portable generator with a floating neutral could compromise the proper operation of GFCIs and OCPDs to the point where they may not operate as designed, thus significantly increasing the risk of electric shock to employees.
Creation of Touch Potential
Secondly, if the ‘hot’ ungrounded conductor contacts the frame of the generator with a floating neutral, all the metal parts will become energized at that electrical potential, including the frame of the vehicle if the generator is mounted to a vehicle. The subsequent ground fault would keep the generator frame energized at hazardous voltages rather than tripping the OCPD and deenergizing the circuit as would occur with a generator with its neutral bonded to the frame.
With the frame of the generator and/or frame of the vehicle energized, there will be a difference of electrical potential between them and the worker who inadvertently touches it. Current only flows when there are differences in electric potential.
Are Floating Neutrals (Ungrounded Systems) Safer?
Some individuals have attempted to argue that an ungrounded system, i.e. floating neutral, is inherently safer than a grounded system because there is no electrical reference to ground. While this may be somewhat true, it only applies to permanently installed ungrounded systems designed and engineered for specific operational needs.
For example, some processing facilities use an ungrounded delta system to provide critical circuits that need a high continuity of operation for systems that can’t be immediately shut down following a single-phase fault to ground fault. The unexpected shutdown of the operations of the equipment could cause damage to both the equipment and/or the product being processed.
In other places, such as nuclear power plants, ungrounded circuits are designed to provide electrical power to critical motor operated valves (MOV) and other important safety related equipment needed for use during emergencies to protect the reactor.
In all such cases, whether the equipment is ac or dc, ungrounded systems are deliberately limited to only very specific needs and for certain key equipment.
Requirements for Ungrounded Systems
Regardless of the reasons for using ungrounded systems, the other important safety fact some individuals miss is that all ungrounded systems are required to have an active ground-fault detection system installed that continuously monitors for ground faults. Most are so sensitive they can detect either a ‘hard ground,’ meaning one phase directly faults to ground with many amps flowing, or a ‘soft ground,’ where milli-amps of current leak to ground due to dampness or other factors that reduce the insulation value.
And it is for this safety reason the National Electrical Code® (NEC®) 250.21(B) mandates the installation of ground-fault detection systems for ungrounded ac systems and 250.157(A) for ungrounded dc systems. If a ground fault occurs, the ground detection system annunciates an audible and/or visible alarm to warn personnel of the dangerous condition so the equipment can be safely powered down to repair the equipment before a second phase faults to ground, which would result in a phase-to-phase short circuit through ground.
No Good for Portable Generators
When using portable generators with floating neutrals, there is no ground fault monitoring system in place to warn employees if a ground fault exists; consequently, the worker is totally unaware of the extremely dangerous condition. Therefore, the argument that ungrounded systems are safer than grounded ones is a moot point when speaking in context of portable generators.
Separately Derived System
Some of you may be wondering , “Why then are portable generators with floating neutrals available?”
To answer the question, we will need to temporarily move away from the main topic to a secondary related one.
Let’s return to the term and definition of a separately derived system, from Part 1, which is defined as “A premises wiring system whose power is derived from a battery, a solar photovoltaic system, or from a generator, transformer, or converter windings, and that has no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system.” The key words from the definition are “that has no direct electrical connection.”
Two excellent examples of separately derived systems would not only include generators but also the utility transformer providing power to your home. They both have two or more sets of windings, which are not physically connected to one another, as shown by Figure 10.