Very true. The question was SAFETY. I hate having 3 wire plugs on
antique electronics. You can easily put a polarized plug on a piece of
zipcord and get the ends right and be safe as well. I like the idea of
making something SAFE without changing the item at all.
-Barry
On 2/17/2012 12:25 PM, Bruce wrote:
Barry, the shock hazard model during HiPot testing development assumes all
external metallic surfaces on a product are always at the same potential.
That is why we are only required to test against the product ground through
an alligator clamp to the chassis or through the ground terminal of the 3
wire AC input. The shock hazard model we care about is a ground path through
the body past the heart. You could be standing on a concrete floor barefoot
or touching a cold water pipe with your other arm.
I submit to you that I could connect any ungrounded electrical device
through my isolation transformer, float it up to 1500VAC (the isolation
voltage of my isolation transformer) and invite you to touch it and
depending on your resistance to ground, you will definitely feel it. It will
not be much current, and it may not kill you but it will wake you up faster
than a few cups of coffee.
Bruce Peterson
Accolade Engineering
www.accoladeeng.com
-----Original Message-----
From: [email protected] [mailto:[email protected]] On
Behalf Of Barry Kasindorf
Sent: Friday, February 17, 2012 8:28 AM
To: Antique Phonograph List
Subject: Re: [Phono-L] Does anyone know how to make a Jones Motrola _safe_?
All this assumes the entire case is at the same potential. Then a shock
occurs if you touch the case and something else that has a non infinite
resistance to the other side of the line at the same time. An isolation
transformer (not a variac or such) that completely isolates the item
from the ground will remove the shock hazard caused by touching the case
of the Motrola and ground. Nothing, including connecting the case to the
green wire of the 3 wire plug will prevent a shock if all the metal is
not at the same potential. If the case halves don't connect well, which
can happen if it is cruddy potmetal you will still get a shock if you
touch ground and the part that is not connected well to the part of the
case you grounded. An isolation transformer will 100% prevent a shock
unless you touch BOTH parts of a faulty case at the same time, and you
will still get a shock if you grounded one part of the case in this case.
The good thing about the isolation transformer is you don't have to make
any changes to the Motrola and it can be used for other electronics when
not using the phonograph.
-Barry
On 2/17/2012 10:51 AM, Bruce wrote:
The isolation transformer does NOT make it safe. We use isolation
transformers when working on TVs/Radios with a hot ground so that when we
connect the ground lead of our instrumentation, we don't see fireworks.
The
isolation transformer allows the isolated circuit to float to whatever
potential the leakage or forced paths take it. It is true that the only
way
to experience a shock is to come in contact with two significantly
difference potentials. I think this discussion of shock hazards already
had
the premise that a potential near ground was available for accidental
contact along with a faulty motrola.
Bruce
-----Original Message-----
From: [email protected] [mailto:[email protected]]
On
Behalf Of Barry Kasindorf
Sent: Friday, February 17, 2012 6:10 AM
To: Antique Phonograph List
Subject: Re: [Phono-L] Does anyone know how to make a Jones Motrola
_safe_?
Not true Greg. You would have to touch 2 different places with potential
to get a shock and the case can't have that.
Unless the isolation transformer is bad it makers it safe. That is how
you can use those AC/DC sets with the case hot safely. The isolation
transformer isolates it from ground completely.
On 2/16/2012 10:10 PM, Greg Bogantz wrote:
An isolation transformer would not prevent getting a shock from the
case of the Motrola if it develops internal leakage. Grounding the
case of the unit as described previously is the best plan.
Greg Bogantz
----- Original Message ----- From: "Philip Carli"
<[email protected]>
To: "Antique Phonograph List"<[email protected]>
Sent: Thursday, February 16, 2012 8:45 PM
Subject: Re: [Phono-L] Does anyone know how to make a Jones Motrola
_safe_?
Many thanks! Another suggestion I've had is the use of an isolation
transformer at the plug - any thoughts on that? PC
________________________________________
From: [email protected] [[email protected]] on
behalf of Greg Bogantz [[email protected]]
Sent: Thursday, February 16, 2012 8:27 PM
To: Antique Phonograph List
Subject: Re: [Phono-L] Does anyone know how to make a Jones Motrola
_safe_?
I believe the Motrola has a metal case. The danger can occur if
there
is electrical leakage from the internal wiring to the case and its
attached
metal parts which can occur due to carbonized insulation that can
begin to
develop a lower resistance. I would recommend first testing with an
ohmmeter to determine if there is already leakage from either of the
wires
leading from the motor to the case. An ohmmeter reading should
indicate a
very high or infinite resistance from either wire to the case when
things
are correct. If you measure any significantly lower resistance, the
internal wiring will need to be redone or repaired. If there is good
isolation from the motor wires to the case, I would recommend
replacing the
line cord with a modern three-wire cord with a 3-terminal AC plug.
Connect
the black and white wires to the motor circuit as was done in the
original
2-wire cord. Then connect the green wire to a screw on the metal case.
This will privide a grounding connection from the case to your household
earthing system. If electrical leakage should develop in the future, it
will be routed thru the green wire to your household ground system
(assuming
you plug the cord into a modern 3-wire outlet). In the worst case,
it will
blow a fuse or circuit breaker rather than leaving the system a shock
hazard.
Greg Bogantz
----- Original Message -----
From: "Philip Carli"<[email protected]>
To: "Antique Phonograph List"<[email protected]>
Sent: Thursday, February 16, 2012 7:08 PM
Subject: [Phono-L] Does anyone know how to make a Jones Motrola _safe_?
I have a Jones Motrola I'm trying to rewire, but I read that they
can be
dangerous in their original ungrounded state? Any ideas on how I
can deal
with this? Any help would be appreciated. Thanks, Philip Carli
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