Neutral Bonding During Island Operation: Exactly One Bond

The reference that makes protection work is established at the service — and the moment a backup source disconnects from the grid, that arrangement has to be reconsidered.

When a battery system or a generator disconnects a house from the grid and
runs it alone, something quiet has to change with it. The reference point that
makes the whole electrical system safe was, until that moment, established at
the service. Once the service is disconnected, that reference has to come from
somewhere else — and it has to exist exactly once.

Neutral Bonding During Island Operation: Exactly One Bond

In a normal grid-connected house, the grounded conductor people call the
neutral is deliberately connected to the grounding system at a single point, at
the service disconnect. That one connection is what gives fault current a
low-resistance path back to its source, which is what makes a breaker open
quickly when something goes wrong rather than leaving metal energised.

Everything downstream of that point keeps neutrals and grounding conductors
separate. That is why a subpanel is not wired like a main panel: a second bond
would create a parallel path, and ordinary neutral current would start returning
through grounding conductors and any metal connected to them. Not a fault — just
current where it does not belong, all the time.

Why islanding disturbs it

An island is a house running from its own source. The question that arises is
whether the original service bond is still doing its job for that source, and
the answer depends entirely on what the transfer equipment opens.

• If the transfer means switches only the ungrounded conductors
  and the neutral stays solidly connected through, the existing service bond is
  still in the circuit and still serving as the system’s single bond. A source
  that brings its own bond would then be a second one.
• If the transfer means also switches the neutral, the
  islanded system is separated from that bond, and it now needs a bond of its own
  or it has none at all.

Neither arrangement is wrong. What is wrong is a combination that produces
two bonds or zero, and the combination is a consequence of two decisions made by
different people — the equipment chosen, and how it was wired.

This is what those generator labels mean

Portable generators are sold in two configurations and the labels are easy to
skim past. A bonded neutral unit has the bond made inside the
machine, which is what you want when appliances are plugged directly into it and
it is the only source in the picture. A floating neutral unit
has no internal bond, which is what you want when it feeds a house through an
inlet and transfer arrangement where the building’s own bond remains in
circuit.

Put a bonded-neutral machine onto a system that already has its bond and you
have two. Put a floating-neutral one onto a system whose bond has been switched
away and you have none. This is the single most common way the subject goes
wrong in practice, and it is decided at the point of purchase — worth settling
before, not after, the equipment arrives, alongside
what a generator inlet installation involves.

What battery systems do about it

Backup-capable battery equipment generally handles this automatically. Many
systems include a bonding contactor or relay that closes the bond when the
system islands and opens it when the grid returns, so exactly one bond exists in
either state. It is a designed behaviour rather than an accident, and it has to
match how the neutral is switched in that particular installation.

That is worth knowing because it means the answer is a property of the whole
design — inverter, transfer equipment, panel arrangement — not of any single
box. It is one more reason the questions in
choosing a battery inverter topology
are about the system rather than the product.

What goes wrong, and how it shows

The second row is the one that deserves attention, because its symptom is
silence. A system with no bond looks and behaves normally right up until the
moment the protection was supposed to work.

The code question is real and it is local

Whether an islanded source counts as a separately derived system, and
therefore what bonding it requires, is written into the electrical standard, and
the edition in force differs across New York: most of the state works under the
state Uniform Code while New York City runs a separate Electrical Code on its
own cycle. Nothing here establishes that any particular installation is correct.
The building department issuing the permit, and the licensed electrician doing
the design, are who determine that.

It is also not a homeowner adjustment. Removing or adding a bonding screw or
strap changes how the system behaves under fault, in a way that is invisible
during normal operation, and the correct configuration depends on equipment and
wiring you cannot assess by looking. The homeowner’s role here is to ask, not to
adjust — the same posture that applies to
bonding at the water service.

What to ask at commissioning

• Where is the system bond, in grid mode and in island mode?
• Does the transfer equipment switch the neutral, or only the ungrounded
  conductors?
• If a generator is part of the plan, is it a bonded or floating neutral
  machine, and which does this arrangement require?
• Does the battery equipment bond automatically when islanding, and was that
  verified rather than assumed?
• Was the system demonstrated in both modes at commissioning, with the
  protective devices tested in each?

That last one is the whole point. A backup system tested only on the grid has
been tested in the state it was not bought for, and the difference will not
announce itself until the night it matters — the night described in
how a storm outage actually unfolds.
Which circuits are live during it is
the subpanel decision,
and whether the protection on them behaves is this one.

Frequently Asked Questions

What is the neutral-to-ground bond for?

It gives fault current a defined low-resistance path back to its source, which is what causes a breaker to open quickly instead of leaving metal energised. In a grid-connected house it is made at one point, at the service disconnect, and everything downstream keeps neutrals and grounding conductors separate.

Why must there be exactly one bond?

Two bonds create a parallel path, so ordinary neutral current begins returning through grounding conductors and any metal attached to them, continuously and not only during a fault. No bond leaves the system without a reference, so fault current has no defined return path and protection may not operate. Both conditions are hazards in different ways.

Why does islanding change anything?

Because the source changes. Whether the original service bond is still serving the islanded system depends on what the transfer equipment opens: if it switches only the ungrounded conductors, that bond remains in circuit; if it also switches the neutral, the islanded system is separated from it and needs one of its own.

What is the difference between a bonded and floating neutral generator?

A bonded neutral machine makes the bond inside itself, which suits appliances plugged directly into it as the only source. A floating neutral machine has no internal bond, which suits feeding a house through a transfer arrangement where the building’s bond stays in circuit. Choosing the wrong one produces either two bonds or none.

Do battery backup systems handle this automatically?

Backup-capable systems generally include a bonding contactor that closes the bond when the system islands and opens it when the grid returns, so exactly one exists in either state. It is a designed behaviour that has to match how the neutral is switched in that specific installation, which makes it a property of the whole design rather than of one product.

How would I know if mine is wrong?

Two bonds tend to announce themselves through ground-fault devices tripping for no apparent reason, or measurable current on grounding conductors. No bond announces nothing at all — the system behaves normally until a fault occurs and the protection does not operate. That silence is why verification at commissioning matters rather than waiting for a symptom.

Can I fix the bonding myself?

No. Adding or removing a bonding screw or strap changes how the system behaves under fault in a way that is invisible during normal operation, and the correct configuration depends on the transfer equipment and wiring rather than on the panel in front of you. The useful homeowner action is to ask for it to be demonstrated in both modes at commissioning.