Earth Grounding Metal Chassis

[Kdog44]

As a rule of thumb, if you have an electrical circuit in a box (metal) that is using the mains voltage, you should always earth ground the box. Is that correct for almost every application?

[Zero999]

Generally yes but even this won't cover you fully.

The resistance between the case and the earth conductor should be under 0.1Ohms. The minimum resistance between live, neutral and earth should be >1MOhm with a leakage current of no more than 1mA.

The earth connection should also be made with a shake proof washer so it doesn't become disconnected.

However there are ways to design a product so the case doesn't need to be earthed which can achieved by insulating all of the mains conductors with reinforced or double insulation, so no single fault can cause the case to be live. If you don't understand this then always earth the case.

[macboy]

Generally yes but even this won't cover you fully.

The resistance between the case and the earth conductor should be under 0.1Ohms. The minimum resistance between live, neutral and earth should be >1MOhm with a leakage current of no more than 1mA.

The earth connection should also be made with a shake proof washer so it doesn't become disconnected.

However there are ways to design a product so the case doesn't need to be earthed which can achieved by insulating all of the mains conductors with reinforced or double insulation, so no single fault can cause the case to be live. If you don't understand this then always earth the case.

This is referred to as doubled-insulated or Class II or Class 2. It is acceptable to have an ungrounded metal chassis on such equipment. A common example is audio equipment, which have metal chassis that are practically never grounded in order to avoid ground loops which cause hum. This is not something to play with since it requires specific construction and testing. For mains powered projects, ground all metal cases, and do it well.

[aveekbh]

Generally yes but even this won't cover you fully.

The resistance between the case and the earth conductor should be under 0.1Ohms. The minimum resistance between live, neutral and earth should be >1MOhm with a leakage current of no more than 1mA.

The earth connection should also be made with a shake proof washer so it doesn't become disconnected.

For mains powered projects, ground all metal cases, and do it well.

The resistance between chassis and earth <0.1 ohm is intended to ensure that fuses/circuit breakers trip immediately if the chassis becomes live and removing the source of dangerous voltages.

In general, any exposed metal should be connected to earth/protective ground. And even then, it is always good practice to double-insulate your mains wiring (use heat shrink on the connectors, put your mains wiring in another tube, etc). This will reduce the chances of exposing mains voltages to the low voltage parts of your circuit.

[Zero999]

Generally yes but even this won't cover you fully.

The resistance between the case and the earth conductor should be under 0.1Ohms. The minimum resistance between live, neutral and earth should be >1MOhm with a leakage current of no more than 1mA.

The earth connection should also be made with a shake proof washer so it doesn't become disconnected.

However there are ways to design a product so the case doesn't need to be earthed which can achieved by insulating all of the mains conductors with reinforced or double insulation, so no signal fault can cause the case to be live. If you don't understand this then always earth the case.

Read "single fault" not "signal fault". Big difference.

Typo, obviously, corrected.