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 >>>> >>>> ________________________________ >>>> >>>> This email message and any attachments may contain confidential >>>> information. If you are not the intended recipient, you are prohibited >>>> from using the information in any way, including but not limited to >>>> disclosure of, copying, forwarding or acting in reliance on the >>>> contents. >>>> If you have received this email by error, please immediately notify >>>> me by >>>> return email and delete it from your email system. Thank you. >>>> _______________________________________________ >>>> Phono-L mailing list >>>> http://phono-l.org >>> _______________________________________________ >>> Phono-L mailing list >>> http://phono-l.org >>> >>> ________________________________ >>> >>> This email message and any attachments may contain confidential >>> information. If you are not the intended recipient, you are >>> prohibited from using the information in any way, including but not >>> limited to disclosure of, copying, forwarding or acting in reliance >>> on the contents. If you have received this email by error, please >>> immediately notify me by return email and delete it from your email >>> system. Thank you. >>> _______________________________________________ >>> Phono-L mailing list >>> http://phono-l.org >> _______________________________________________ >> Phono-L mailing list >> http://phono-l.org -- -Barry _______________________________________________ Phono-L mailing list http://phono-l.org _______________________________________________ Phono-L mailing list http://phono-l.org
