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Here says that devices with metal shells usually use three-prong plugs.

Three-prong plugs are for appliances that require the ground connection for safety. Most appliances that use a metal chassis require a separate ground connection.

I am confused about this. My Macbook Pro uses alumnum case, but why it can still use two-prong plugs? Will it increase the risk of electric shock?

Does anyone have ideas about this? Thanks!

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closed as off-topic by brhans, Voltage Spike, RoyC, uint128_t, Dave Tweed Feb 14 '18 at 19:52

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions on the use of electronic devices are off-topic as this site is intended specifically for questions on electronics design." – brhans, Voltage Spike, RoyC, uint128_t, Dave Tweed
If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ See superuser.com/questions/461898/… \$\endgroup\$ – Passerby Feb 13 '18 at 7:15
  • \$\begingroup\$ Reading the For Dummies books gets you only that far, doesn't it? \$\endgroup\$ – Nick Alexeev Feb 14 '18 at 0:06
  • \$\begingroup\$ Class 2 isolation. \$\endgroup\$ – winny Feb 14 '18 at 11:10
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The Macbook has an external power supply that converts AC to DC. The power supply provides the necessary safety and isolation, so you don't need earthing of the Macbook case.

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  • \$\begingroup\$ Yes. Useful search term : "double insulation". \$\endgroup\$ – Brian Drummond Feb 13 '18 at 11:47
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The macbook is a class III appliance, meaning that it is powered by a safety extra-low voltage supply. The low voltage guarantees that coming to contact with energized parts poses no risk in normal circumstances.

The actual appliance from a safety perspective isn't the macbook, but the power brick. The line input must be either

  • Double insulated (class II appliance) from all user accessible parts of the device, including the low voltage output. The idea is to guarantee that any fault cannot make the chassis live, and this is accomplished with sufficient isolation. No safety earth lead is needed.

  • Single insulated, but with the chassis (all user accessible conductive parts) earthed (class I appliance). The idea is that any fault is shorted to ground, keeping the chassis of the appliance at a safe potential. A three prong plug obviously required, and such a power supply should only be used with grounded outlets.
    This approach also makes suppressing electromagnetic interference simpler.

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Power adapters (with AC high voltage inputs) that are rated Class 2 are safe without grounding, and (of course) a MacBook Pro is battery operated and must operate without ground connection, or it wouldn't work at all.

A laptop computer has low voltage (battery) power, and the small amount of HV supplied to its backlights is 'isolated' so a ground connection wouldn't complete a circuit (thus, wouldn't protect). That's OK, because a grounded human also wouldn't complete a circuit, and wouldn't get a shock.

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