Hi As I understand it, 110v power tools are considered safer because the 110 volt supply is Center Tapped to Earth, so any shock would be 55 volts - I'm not sure exactly how this works - could someone explain please? Does this only apply to universal (brush) motors?
Secondly, I also understand that a power tool with an induction motor (saw bench, pressure washer) in 110 volt can't be CTE, so any shock would be
110v. Is that right?Dave
They run off a transformer with 110V secondary, the centre of which is earthed. This means (ignoring RMS issues) one conductor is +55V and the other is -55V, and every 10ms they swap around as it's AC.
So when some building site labourer drops a drill into a puddle on a construction site as he's about to use it, the maximum electric shock he's likely to get to earth is 55V which is very unlikely to e harmful.
No. It applies to all contruction site tools up to 3kW. (Actually, following EU rulings, it's no longer mandatory in the UK.)
No. There's no reason an induction motor cannot also run from a
110V centre tapped earth supply.There's also a 3-phase equivalent for higher power construction site tools, and some induction motors are likely to use this.
This centre-tap to earth business works when feeding a device from an isolating transformer. Since the secondary of the transformer is isolated from earth, unlike one side of the mains supply that feeds it, you can choose to reference the secondary to earth from a centre tap if available. The centre of the transformer secondary is now tied to the mass of the earth, so the ends of the secondary only ever rise 55V rms above earth potential. Imagine a seesaw with the centre pivot being the reference point: each side will go up and down, but only half the distance compared with if you held one end still and let the free end waggle up and down.
Any shock you are likely to get will be between one end of the secondary supply and earth ( since you are permanently connected to earth via your feet ), so you will probably only get a 55V rms shock.
I don't believe the sort of motor you are feeding has a bearing on the application of this safety method. If someone else knows better I'm sure they'll post.
Andy.
Is the secondary N of the big yellow 110v site transformers bonded to the earth of the primary, or is it just provided to the earth & N terminal of the 110v socket?
There is no secondary N. There are two phase (live) conductors and earth.
Interesting! When did that change Andrew?
Dave
There's no neutral terminal in the 110V socket, just two phases and earth - like this:
L >----)|(--------( L1 )|( )|( )|( 230V )|(--- 110V )|( | )|( | )|( | N >----)|(--------( L2 | | | E >----------*----( E
The 110v is derived from an isolating transformer which removes any reference to earth that the 240v supply has. In other words you could safely touch either output line as neither has a reference to earth. But of course touch both, and you'd get 110 volts which is still a dangerous voltage. If you then connect the centre tap of the output winding to earth, you'll get a maximum of 55 volts from either line to earth which is safe for most, but still of course 110 across the lines.
IMHO, this dates back to when most on site tools had metal casings which had to be earthed. Using modern double insulated tools would suggest to me the 110 would be best left 'floating'
No - any electric motor etc can be made for 110v. The only difference is it will take double the current a 240v one would, and need heavier cable.
The CTE makes no difference - the supply is still 110 volts.
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