The ultimate noobie question

[MortimerJazz]

I feel a bit of a muppet for asking this, but here goes anyway.

I seem to have got two power adaptors with my MacBook. The one with the big white box plugs nicely into the wall and into the side of the Mac, which is great.

However, I've also got another cable which has a figure of 8 connection at one end of it. And I can't for the life of me figure out where that plugs in to. What's the point of this cable?

Thanks a load in advance

 

[EricNau]

You can pull the plug off of the power brick (pull straight up, not out) and replace it with the cord, grounding your connection and adding length.

Your instruction manual explains this very nicely.

 

[bartelby]

You can pull the plug off of the power brick (pull straight up, not out) and replace it with the cord, grounding your connection and adding length.

Your instruction manual explains this very nicely.

Correct except the grounding part. The additional cable is only 2 core wire; Live and neutral. No earth.

 

[richard.mac]

Correct except the grounding part. The additional cable is only 2 core wire; Live and neutral. No earth.

so neither the brick plug or the cable plug have a third earth prong? is this because neither the macbook or macbook pro have any conducting materials on the casing i.e. plastic and aluminium?

 

[bartelby]

so neither the brick plug or the cable plug have a third earth prong? is this because neither the macbook or macbook pro have any conducting materials on the casing i.e. plastic and aluminium?

Not sure, could be. Lots of electrical goods only have live and neutral connections.

 

[EricNau]

Correct except the grounding part. The additional cable is only 2 core wire; Live and neutral. No earth.

Are you sure? Apple says, "The AC cord provides a grounded connection" and "For best results, always use the power adapter and connect it to a grounded power outlet when one is available," although both statements are somewhat vague. Link

Plus, Wikipedia's MagSafe page lists the two outer pins as grounds.

Based on what I've been told by Apple techs, using the included 3-prong cord is recommended for grounding purposes. Then again, I'm no electrical engineer, and neither are they.

 

[bartelby]

Are you sure? Apple says, "The AC cord provides a grounded connection" and "For best results, always use the power adapter and connect it to a grounded power outlet when one is available," although both statements are somewhat vague. Link
.

It terminates in a standard "euro" plug. Unless Apple modify thm to take a 3rd connection there's no earth. I can't check as my laptop is being repaired and they have the power cord.

Edit: I've just found my old burned out power brick. You could be right. The pin that supports the plug/power cord is metal. So if the little end, of the power cord, has a metal insert it will ground.

This is why I love Apple. They have elegant solutions to things and you don't even notice.
Apple designers go up another notch for me.

 

[richard.mac]

hmm well i just looked at both my magsafe and cinema display cord plugs as i was lazy and they both have a ground prong so you where right EricNau.

i was just interested to askl. it seems the magsafe brick plug doesnt have a ground prong so it would be wise to use the magsafe cord if using a macbook as a desktop replacement which i do.

 

[EricNau]

It terminates in a standard "euro" plug. Unless Apple modify thm to take a 3rd connection there's no earth. I can't check as my laptop is being repaired and they have the power cord.

Could the round metal "clasp" on the adaptor be used to carry the ground?

There's definitely a third prong on the included plug, which typically indicates a grounded connection in the states.

 

[Makosuke]

Then again, I'm no electrical engineer, and neither are they.

I, however, occasionally pretend to be one at work.

And you're correct--at least on US adapters, the long cable with the 3-prong plug DEFINITELY adds a ground. You can see the contact if you look in the slot, and that's why the sort of button thing that locks the connector in place is metal.

I just checked mine with the meter I have at home, and while there's something in between adding some resistance, the ground pin on the plug is definitely connected to the two outside pins on the magsafe connector, which are in turn connected to the metal frame in the MBP--there's a current path between the ground pin and any of the exposed metal (screws or the USB shield, for example--the palm rest area isn't conductive, I guess because of the paint?).

I'm not going to take my adapter apart to find out in any more detail than that, but using the 3 prong cable does increase safety at least somewhat.

[Edit: Whoops, three posts since I started typing--and yes, EricNau, that round metal clasp is what carries the ground.]