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

In article ,
"david lang" writes:
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?

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.

Does this only apply to universal (brush) motors?

No. It applies to all contruction site tools up to 3kW.
(Actually, following EU rulings, it's no longer mandatory in the UK.)

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?

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.

--
Andrew Gabriel

"david lang" wrote in message
. ..
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

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.

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.

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?

In article ,
"Sparks" writes:

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.

--
Andrew Gabriel

Andrew Gabriel wrote:

No. It applies to all contruction site tools up to 3kW.
(Actually, following EU rulings, it's no longer mandatory in the UK.)

Interesting! When did that change Andrew?

Dave

Sparks wrote:

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's no neutral terminal in the 110V socket, just two phases and
earth - like this:


L ----)|(--------( L1
)|(
)|(
)|(
230V )|(--- 110V
)|( |
)|( |
)|( |
N ----)|(--------( L2
|
|
|
E ----------*----( E


--
Andy

In article ,
david lang wrote:
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?

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'

Does this only apply to universal (brush) motors?

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.

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?

The CTE makes no difference - the supply is still 110 volts.

--
*Can fat people go skinny-dipping?

Dave Plowman London SW
To e-mail, change noise into sound.