Thread: Grounding pins on non-metallic appliances

Greetings all - can anybody explain why there are sometimes grounding pins on plastic-bodied appliances? Isn't the point of the ground to prevent a conductive appliance body from becoming electrified?

And if I'm off-base in the above paragraph, can someone explain exactly what the criteria is for requiring a ground? It seems at least somewhat arbitrary - for example, I have two laptops. One's power supply has a ground pin and the other one doesn't. They are both made of plastic of course. Could it have something to do with the output voltage?

Thanks!

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Originally Posted by thaddeus cox

Greetings all - can anybody explain why there are sometimes grounding pins on plastic-bodied appliances? Isn't the point of the ground to prevent a conductive appliance body from becoming electrified?

And if I'm off-base in the above paragraph, can someone explain exactly what the criteria is for requiring a ground? It seems at least somewhat arbitrary - for example, I have two laptops. One's power supply has a ground pin and the other one doesn't. They are both made of plastic of course. Could it have something to do with the output voltage?

Thanks!

The grounding of a laptop power supply has nothing to do with the output voltage.

The main component would be a simple stepdown transformer which only requires a 2-wire supply, the hot and the neutral. But there is also a little circuit board in there that regulates the supply to the laptop, and the grounding conductor will help to prevent stray voltages from drifting around in there and frying fragile microchips. That's my theory, anyway. Yes, the outer body is plastic, but the ground connection is to the circuit board inside.

The output from the power supply to the laptop of course is only two wires, + and - DC.

It might be as simple as the manufacturer only wants to stock one cord type. They probably have some units (e.g. for desktop PCs with a metal shell) that require a ground and it costs less for them to just provide a cord w/ a ground pin than to stock two different kinds of cords.

Matt

I would expect most any electronic device that uses high speed switching to generate a lot of RF noise and harmonics if there wasn't a power ground. Plus, if the power supply has any effective surge protection built into it, it would have to have a ground.

So noise and surge suppression aside... is there any reason to have a ground on a non-conductive-bodied appliance?

Grounding is not only about about protecting us from electrical shock. As for your lap top computer power supply having a ground pin, here is an example. Just about all electronic devices are powered from switching power supplies. These type of power supplies do not use a standard step down transformer. The old power transformers are not very energy efficient because they took the 120 volts AC - 60 Hz and steped the voltage up or down. The output current ( not voltage ) was based on the size of the secondary wire. The voltage ( not current ) was based on the number of turns of wire on the secondary side of the transformer. The secondary current was then rectified, filtered, and regulated to the necessary voltage the device needed. Todays electronics use what is called a switching power supply. These power supplies take the 120 volt AC - 60 Hz and rectify it to DC. It's then filtered to a clean DC voltage. This voltage feeds an oscillator circuit which changes the DC to AC at about
1000 Hz. The output of the oscillator is coupled back to the input side of the oscillator to controll the amount of current flowing through the circuit. The voltage is then steped down via a transformer, rectified, filtered and regulated to supply a clean DC output. By now your asking how the heck this can be more efficient. I can get into alot more detail but to keep it as simple as I can, the higher the Hz or cycles per second, the smaller the transformer needs to be and the more power efficient the power supply becomes. The power supply has the capability of sensing the amount of current needed to power a device and creates no more or no less then needed. Hence the word "switching". When the power supply senses a load or current draw, the output of the oscillator controlls the input of the oscillator to maintain the current needed. The voltage is regulated to maintain a constant voltage. The outer housing of these power supplies are plastic, however, the circuit is usually housed in a metal container and is grounded to protect stray RF from interfering with other devices. I hope this answers your question.

Originally Posted by thaddeus cox

So noise and surge suppression aside... is there any reason to have a ground on a non-conductive-bodied appliance?

Consider a hand-held mixer made with a plastic body. The frame which carries the motor is made from a cast metal, which in a fault condition, may become energized. The metal frame is connected to metal beaters which could come into contact with a human hand. The idea is to ground any non-current carrying part, which may become energized in a fault condition, so that the noncurrent-carrying part remains at earth potential (same as a human standing on a floor).

By design, intent and function under normal operation, the ground wire does nothing. It is like E&O insurance in that it is there to protect you in the event of a fault, but hopefully never needed. Even with non-conductive exteriors there are conductive components inside / close or electricity would not be involved. Using the ground wire as means to bleed off undesired transients could be a design function, but designed correctly, it again should not be needed.

Originally Posted by thaddeus cox

Greetings all - can anybody explain why there are sometimes grounding pins on plastic-bodied appliances? Isn't the point of the ground to prevent a conductive appliance body from becoming electrified?

With regard to grounding for personnel protection - if the plastic bodied appliance was "double insulated" (not just an outer plastic case), then the equipment grounding conductor is not required.

"Double insulated" does not mean you cannot receive an electrical shock from the equipment faulting out, only that a single fault will not result in the potential for an electrical shock, however, total failure might lead to the potential for an electrical shock (of course, total failure could mean it is now on fire, in which case you would likely not continue to hold on to that appliance).