Grounding and bonding get said as one phrase and treated as one idea, which is why neither gets understood. They are two different jobs solving two different problems, and a house can have one working properly while the other has been quietly defeated for decades.
Grounding: a path to earth
Grounding connects your electrical system to the physical earth, usually through a rod driven into the soil outside or a clamp on the metal water line where it enters the house.
Its job is not, as most people assume, to carry fault current away into the ground. Soil is a poor conductor. What it actually does is hold your system at the same reference voltage as the earth around it, so that a surge from a lightning strike or a utility fault has somewhere to go other than through your wiring, and so the voltage on a wire means the same thing everywhere in the house.
Bonding: everything at the same voltage
Bonding connects the metal parts that are not supposed to carry current — the panel enclosure, water and gas piping, the frame of your furnace, a metal outlet box — to each other and back to the system.
The reason is voltage difference. You do not get a shock from touching something energised; you get a shock from touching two things at different voltages at once. Bonding makes sure that if a live wire touches a metal casing, that casing and the tap you are holding are at the same potential, and the fault current has a low-resistance path back to the panel that trips the breaker instead of a high-resistance path through you.
This is the half people break by accident. Replacing a section of metal water pipe with plastic can sever a bonding path that has been in place since the house was built, and nothing will look wrong afterwards.
Why older houses are the problem
Knob-and-tube wiring has no ground conductor at all — it was installed before the concept was standard, so there is nothing to bond an outlet to. That is why an older home often has two-prong receptacles: not because someone was cheap, but because there is no third wire to connect.
The dangerous version is a two-prong outlet that someone replaced with a three-prong one without running a ground. It now accepts a grounded plug and looks correct, while the ground pin connects to nothing. Every surge protector plugged into it is inert, because a surge protector works by diverting energy to ground.
What good looks like
A continuous path from every metal part back to the panel, a solid connection from the panel to at least one earth electrode, and no interruptions introduced by later renovations. It is not visible from a light switch, which is why it is one of the first things an electrician checks and one of the last things a homeowner would think to.
How to spot it in your house
- Look for a bare copper wire clamped to your water line near where it enters the house, or running out through the foundation to a rod.
- Two-prong receptacles anywhere in the house mean those circuits are ungrounded.
- A three-prong receptacle in a house with knob-and-tube or early aluminum wiring is worth testing — a plug-in outlet tester from any hardware store will tell you whether the ground pin is connected to anything.
- Plastic sections in an otherwise metal water line can mean a bonding path was cut during a plumbing repair.
The next question
Older-home wiring: knob-and-tube and aluminumRelated terms
- GFCIA device that cuts power within milliseconds if current starts leaking to ground, through water or through a person. Required near sinks, outdoors and in bathrooms.
- Knob-and-tubeWiring used in houses built before roughly the 1950s, with no ground wire and cloth-and-rubber insulation that hardens and crumbles with age.
- BreakerA switch in your panel that cuts a single circuit when it draws more current than the wire behind it can safely carry.