Why no ground wire?

[skippyelwell]

I've been taking apart and restoring electric motors on machines I'm working on for years, with motors more than say 30 yrs old, there is almost never a ground wire coming from the motor. True of both single phase and 3ph, what's the deal?
Are newer codes just safer?

 

[Arbutus]

The absence of a ground wire in older electric motors, whether single-phase or three-phase, is largely due to the evolution of electrical safety standards over the past several decades.
Many motors built more than 30 years ago were designed before modern grounding requirements were widely adopted. Electrical codes like the National Electrical Code (NEC) in the U.S. have progressively emphasized the importance of grounding for safety, but this was not always a priority.

In older systems, the motor's metal frame was often considered sufficiently grounded through its physical mounting to the machine or via the metallic conduit enclosing the wiring. This was considered acceptable because it provided a path to earth in the event of a fault. Early designs assumed that insulation materials and physical barriers provided adequate protection from faults. If insulation failed, the belief was that fault currents would safely travel through the motor casing and connected metal structures.

Many three-phase motors from that era lacked a dedicated ground wire because grounding was integrated into the larger electrical system. The metallic conduit and equipment frames were expected to serve as the ground path.Electrical systems were often ungrounded or corner-grounded, especially in industrial setups, which reduced the emphasis on individual motor grounding.

Older single-phase motors, such as capacitor-start or shaded-pole designs, were often used in applications where safety was less critical, like home appliances or tools. Grounding the motor wasn't deemed essential because the systems they were connected to were simpler, often relying on double-insulation or similar methods.

All that aside, theres no reason not to add a ground wire explicitly from the motor to the electrical box.

 

[skippyelwell]

Thank you sir, it's a question I've been meaning to look in to for years but by the time I get from the shop to the computer it has evaporated.
Yes, I do add ground wires to older motors every time I come across it, I know that everything I own will belong to someone else at some point and those people may not be as immortal as I like to think I am.

 

[mbond]

For those of us who live in the city, ground wires look like an added layer of safety. But even in the city, if you look carefully at the power poles, you will see that ground and neutral are merged. And then they go nowhere except into the ground

 

[djberta]

In the 2 prong era there was no grounding, what we refer to as bonding now. There was no bond as Arbutus described perfectly. In the 2 prong era there was not even polarization for the ungrounded (hot) and the grounded (Identified conductor, originally known as the neutral). The idea was that the grounded conductor being neutral would take stray or short circuit currents back to the panel to melt a fuse or throw a breaker. They kind of did. But they killed people. Just like when automobiles had no safety measures deaths were considered like a cost of usage. This changed over time. We are still working on the death being the costs of construction, we have gotten a lot better but we are still killing people.

One of the key changes to code in Canada and the US in the last couple of decades is dispelling the idea that the white wire is neutral it is only neutral in a balanced 3 wire circuit. Which is a very rare thing in most single phase wiring. It is far more prevalent for the identified wire to be carrying current.

 

[mbond]

Every 3 phase system carries the largest current in the 'zero current' white neutral wire.

 

[djberta]

Every 3 phase system carries the largest current in the 'zero current' white neutral wire.

This exactly.

Back to the OP's comment. Why we refer to the white wire as the identified grounded conductor. Bonds are called the grounding conductor. Hots are called the ungrounded conductors. Trying to change 100 years of describing something is going to take time though.

 

[mickeyf]

ground and neutral are merged

the white wire is neutral it is only neutral in a balanced 3 wire circuit. Which is a very rare thing in most single phase wiring

Also, while ground and neutral are connected at your panel, they diverge and the further the wire travels from the panel the greater the differential. Eventually, it may be a large enough difference to effect safety.

 

[mbond]

This exactly.

Back to the OP's comment. Why we refer to the white wire as the identified grounded conductor. Bonds are called the grounding conductor. Hots are called the ungrounded conductors. Trying to change 100 years of describing something is going to take time though.

I have never heard that specific terminology, but it seems about right for the standard wiring in North America.

for 3 phase systems, you have the choice of Y or delta wiring. Both could be with or without a ground, but only the Y layout has a neutral. The delta arrangement is almost always used for long distance transmission, and the Y arrangement is typical for short distances (within a single building or floor)

for single phase, you have the choice of hot - hot (phase to phase) or hot neutral. For standard residential, hot - hot will be either 240v or 208v nominal (depending on the transformers on the street - they could be split tap or 3 phase) and hot - neutral is 120v nominal.

In all of these cases, neutral means that the wire will carry current, but is not energized from a power source. It goes to a grounding rod or some other sink that the energized wire(s) push electrons into, and pull them back out of as the voltage alternates. In a single phase hot-neutral system, the current in both is balanced except in the case of a fault. Ground fault breakers work by checking that the current in each is balanced for example

For all of these designs, the addition of a dedicated ground wire is designed to provide an alternate path for electrons from the energized conductor to a ground sink in case something fails. These wires do not normally carry any current, and it is expected that all of the housings (electrical boxes etc.) will be connected together to provide a level of redundancy in the path to earth. Isolated ground systems are different.

Grounding provides an added level of safety principally in the case of mistakes. Mis wired equipment, loose conductors, voltage spikes and anything else that can generate an arc. Worn insulation or bad environmental conditions too. The grounding wires themselves don't intrinsically add more safety, but the idea is to add more redundancy in the safety failback path because people make mistakes