Lessons · Electrical · grounding and bonding

Grounding connects to the earth; bonding connects the metal together

Grounding ties the electrical system to the earth at one point, so voltages have a reference and lightning has somewhere to go; bonding ties every piece of metal that could become energised back to the source with a low-resistance path, so a fault becomes a big current that trips the breaker instead of a live box waiting for a hand.

Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.

What it is for

A hot wire chafes through inside a metal junction box. If that box is bonded, hundreds of amps flow and the breaker opens in a heartbeat. If the box is only tied to a rod in the dirt, a few amps flow, the breaker never notices, and the box sits at 120 V until the next person touches it. Two different words because they do two different jobs.

How to think about it

Ask of any metal part: if a hot touched this, where would the current go and how much would flow? Bonding is about the path back to the source, low resistance, through the green or bare wire. Earth is a poor conductor; a ground rod alone will not trip a breaker. Before touching any bonding jumper or ground bar, de-energize and test.

Worked example

Fault: hot to a bonded metal box, loop resistance 0.4 Ω

The equipment grounding conductor back to the panel, and the neutral bar to the source.

I = 120 / 0.4 = 300 A

Fifteen times a 20 A breaker's rating. It opens instantly.

Same fault, box tied only to a ground rod, 25 Ω to earth

A good rod on a good day.

I = 120 / 25 = 4.8 A

The breaker sees 4.8 A of a 20 A rating and does nothing. The box is live at 120 V.

Your turn

A fault through a bonding path of 0.6 Ω on 120 V. Write the current.

I = 120 /  = 200 A

The trap

Calling a ground rod a safety ground. The rod is for lightning and for holding the system near earth potential. What clears a fault is the bonding conductor back to the panel, and a box with no green wire is not protected by any amount of dirt.

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