Key takeaways
- Industrial grounding and bonding are covered by NEC Article 250, for the service, the equipment and the structure.
- Bonding gives fault current a low resistance path back to the source so the breaker or fuse opens quickly.
- Each step-down transformer is a separately derived system and needs its own bonding jumper and grounding electrode connection.
- Plants that grew piece by piece often have gaps in bonding, and those are found by checking rather than assuming.
Industrial grounding and bonding decide where current goes when something fails. Done well, a fault clears in an instant, equipment frames stay at a safe voltage and controls stay quiet. Done poorly, a fault can leave a machine frame energized, a drive can trip for no clear reason, or a surge device can have nowhere to send its energy. This guide explains how grounding and bonding work in a plant, where they usually fall short, and how we check and correct them in San Antonio facilities.
What is the difference between grounding and bonding?
Grounding connects the electrical system to the earth through grounding electrodes, which helps limit voltage from lightning, utility surges and contact with higher voltage lines. Bonding connects metal parts to each other and back to the source, so fault current has a low resistance path and the breaker or fuse opens quickly.
People often use the two words as if they meant the same thing, but they do different jobs. The earth is not a good path for fault current, so grounding alone will not trip a breaker. Bonding is what does that: every raceway, enclosure, tray section and equipment frame tied together and back to the service or transformer, so that a fault from a conductor to a frame becomes a short circuit the protective device sees at once.
The two systems meet at one point: the service, or the transformer for a separately derived system. There the neutral, the equipment grounding conductors and the grounding electrode conductor come together through a bonding jumper. Everywhere downstream of that point, the neutral and the grounding conductors are kept apart. Most of the grounding and bonding problems we find in plants come down to that one rule being broken somewhere along the way, usually during an addition made in a hurry, and the symptoms rarely point back to the panel where the mistake sits.
Grounding is one of the core services on our industrial electrical contractor page, and the same principles apply at smaller scale in our grounding installation work.
Why does grounding and bonding matter so much in a plant?
A plant has more metal, more motors and more electronics than most buildings, and people touch that metal all day. Good bonding keeps a fault from leaving a machine frame energized. Good grounding helps surge devices and drives work as intended and keeps noise from upsetting controls and network equipment.
Drives, control panels and network gear are all sensitive to small voltage differences between pieces of equipment that are connected by control or data cables. When bonding is incomplete, those differences show up as signal errors, failed communication ports or nuisance trips. Surge protective devices also depend on the grounding behind them: a device with a poor connection to ground cannot do its job.
Drives are one of the loads most affected, as our guide to VFD installation explains.
Which parts of a plant have to be grounded and bonded?
Under NEC Article 250, the service, the equipment and the structure all have to be grounded and bonded. That includes the grounding electrode system at the service, each separately derived system, every raceway, cable tray and enclosure, motor and machine frames, and metal piping and structural steel where the code calls for bonding.
Cable tray deserves its own mention. NEC Article 392 covers how tray is bonded, and in a plant where tray carries many circuits, a tray run with a broken bond is a long piece of metal that may not be part of the fault path at all. Flexible connections at vibrating equipment are another weak point, because the flexible section is not always a reliable bonding path on its own and may need a separate bonding conductor.
How are transformers grounded inside a plant?
Each step-down transformer creates a separately derived system. Its secondary needs one system bonding jumper connecting the neutral to the grounding system, and a grounding electrode conductor to the building's grounding electrode system, both made where the code describes. Missing or duplicate connections put neutral current on the grounding path.
This is one of the most common faults we find. A transformer added years ago may have its neutral bonded at the transformer and again at the panel it feeds, which splits normal neutral current onto the equipment grounding conductors and raceways. Or the bond may be missing entirely, which leaves the secondary without a proper fault path. Either way, the fix is to make the bond in one place, as Article 250 requires, and confirm the grounding electrode connection.
Transformers and the systems they feed are covered in more depth in our guide to 480 volt distribution and transformers.
What problems point to a grounding or bonding fault?
Common signs include current measured on grounding conductors or raceways during normal operation, mild shocks from equipment frames, drives or controls that fault for no clear reason, communication errors between connected equipment, surge devices that fail repeatedly, and breakers that are slow to clear a fault or do not trip at all.
None of these proves a grounding problem on its own, since loose connections, harmonics and failing parts can cause similar symptoms. That is why we measure before we change anything. A clamp meter reading on a grounding conductor, a look at where neutrals and grounds are joined, and a walk of the bonding path from a piece of equipment back to its source usually show whether grounding and bonding are part of the problem.
How is grounding and bonding checked in a plant that grew piece by piece?
We start at the service and work outward: the grounding electrode system, the main bonding jumper, each transformer's bonding and electrode connection, then feeders, panels, tray and equipment. Each joint along the path is checked, and gaps are corrected under NEC Article 250 and recorded on the drawings you keep.
Plants that have grown over many years often carry the habits of every electrician who worked there. A machine added in a hurry, a tray extension with no bonding jumper, or a panel where neutrals and grounds share a bar it should not share are typical finds. Most of the checking happens while the line runs. Corrections that need equipment off, such as rework inside a panel or at a transformer, go onto the work list for a planned shutdown.
Those corrections fit naturally into our industrial electrical maintenance program, and new runs are bonded as part of our industrial electrical wiring work.
When should grounding and bonding be corrected?
As soon as a fault is found that could leave a frame energized or keep a breaker from clearing, since those are safety problems. Other gaps, such as a missing tray bond or a doubled neutral bond, should be corrected at the next planned shutdown, and every new installation should be bonded correctly from the start.
Grounding and bonding also deserve a look whenever major work is planned: a new transformer, a service change, a new process line or a set of drives. It is far easier to correct the grounding while the equipment is open for other work than to come back for it later.
Where an arc flash study is in place, changes to transformers or the service are flagged to the engineer, as our post on arc flash labels explains. For food processing and manufacturing floors, see our manufacturing electrician page, and for the rest of our scope, commercial electrical services. To have your plant's grounding and bonding checked, reach us through our contact page.