Negative rail grounding
Locked
 
|
I though I would separate this from the other discussion. It seems that my newest Astron SS30 power supply does not allow for the negative post to be isolated from the chassis. The green wire can be disconnected from the chassis but the negative rail of the DC output can not be isolated from the chassis. The negative rail is firmly connected to the chassis via two of the circuit board mounting screws. Separation would only be possible by using insulating washers and nylon screws.
This is not true in one of my older supplies. Has there been an NEC or UL change that requires that one side of the output of a DC supply be grounded? Michael Blake [hidden email] ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
|
|
On 4/18/2018 11:10 AM, Michael Blake wrote:
> It seems that my newest Astron SS30 power supply does not allow for the negative post to be isolated from the chassis. The green wire can be disconnected from the chassis but the negative rail of the DC output can not be isolated from the chassis. The negative rail is firmly connected to the chassis via two of the circuit board mounting screws. Separation would only be possible by using insulating washers and nylon screws. That's unfortunate, and a good reason to avoid that supply. It IS an absolute requirement of virtually all Electrical Codes (LAW) in North America that the Green Wire be bonded to the shielding enclosure (chassis). I suggest that you check that paint is not insulating the mounting screws fro the shielding enclosure. > > This is not true in one of my older supplies. Has there been an NEC or UL change that requires that one side of the output of a DC supply be grounded? NEC does not apply to low voltage DC systems. 73, Jim K9YC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
|
|
Jim, I am not sure why you suggested I check for paint under the mounting screws. IT IS bonded to the chassis and cannot be isolated.
I did not believe that the NEC applied but I was not sure about UL. Michael Blake [hidden email] > On Apr 18, 2018, at 14:28, Jim Brown <[hidden email]> wrote: > >> On 4/18/2018 11:10 AM, Michael Blake wrote: >> It seems that my newest Astron SS30 power supply does not allow for the negative post to be isolated from the chassis. The green wire can be disconnected from the chassis but the negative rail of the DC output can not be isolated from the chassis. The negative rail is firmly connected to the chassis via two of the circuit board mounting screws. Separation would only be possible by using insulating washers and nylon screws. > > That's unfortunate, and a good reason to avoid that supply. It IS an absolute requirement of virtually all Electrical Codes (LAW) in North America that the Green Wire be bonded to the shielding enclosure (chassis). I suggest that you check that paint is not insulating the mounting screws fro the shielding enclosure. >> >> This is not true in one of my older supplies. Has there been an NEC or UL change that requires that one side of the output of a DC supply be grounded? > > NEC does not apply to low voltage DC systems. > > 73, Jim K9YC > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[hidden email] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to [hidden email] Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
|
|
In reply to this post by Michael Blake
There's no safety requirement that DC return be connected to chassis.
SS30 is an SMPS so a different beast than the prior discussed linear supplies. It's possible that the DC return to chassis connection is needed for EMC compliance. That's providing a low impedance return path for noise generated by switch elements. Without the connection, noise will take an unwanted path and come out as common mode noise. A frequent source of the noise is capacitive coupling from MOSFETs to chassis, sometimes via a heat sink. I would be hesitant to simply insulate the DC neg from chassis. One possibility would be to cut the copper around PCB mounting screws (carefully!), and put a cap between them so you maintain an AC connection but block the unwanted DC path. You want the capacitor value to be much greater than the parasitic capacitance. 73, Josh W6XU On 4/18/2018 11:10 AM, Michael Blake wrote: > I though I would separate this from the other discussion. It seems that my newest Astron SS30 power supply does not allow for the negative post to be isolated from the chassis. The green wire can be disconnected from the chassis but the negative rail of the DC output can not be isolated from the chassis. The negative rail is firmly connected to the chassis via two of the circuit board mounting screws. Separation would only be possible by using insulating washers and nylon screws. > > This is not true in one of my older supplies. Has there been an NEC or UL change that requires that one side of the output of a DC supply be grounded? ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
|
|
On 4/18/2018 1:33 PM, Josh Fiden wrote:
> SS30 is an SMPS so a different beast than the prior discussed linear > supplies. It's possible that the DC return to chassis connection is > needed for EMC compliance. That's providing a low impedance return > path for noise generated by switch elements. Without the connection, > noise will take an unwanted path and come out as common mode noise. In fact, the opposite is true. When V- is NOT bonded to shack ground, the power cable forms a transmission line, confining the field for noise current to a small region around the pair. But when V- IS bonded to shack ground, return current divides between the shack ground path and the V- (black) conductor. The fields no longer cancel, the noise current takes whatever path it chooses, and a magnetic loop is formed between V+ and shack ground. Common mode noise current can easily be blocked using ferrite chokes on the DC cable just was we do with coax. > A frequent source of the noise is capacitive coupling from MOSFETs to > chassis, sometimes via a heat sink. Yes. Interior circuit design and layout is critical to low RF noise. 73, Jim K9YC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
|
|
In reply to this post by Jim Brown-10
On 4/18/2018 11:28 AM, Jim Brown wrote:
> NEC does not apply to low voltage DC systems. Oh? Check NEC Section 7 , specifically Article 720, which applies specifically to Low Voltage systems. (We do a LOT of 48V distribution system designs.) Philip M. Kane Esq / P.E. (K2ASP since 1952) VP - General Counsel & Engineering Manager CSI Telecommunications, Inc. - Consulting Engineers San Francisco, CA - Beaverton, OR ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
Re: Negative rail grounding
|
Administrator
|
Gentlemen, This thread and related threads is getting well past our max OT posting limit. Please take it off list for now and report back on your final conclusions if you come to agreement.
73, Eric Moderator most of the time.. elecraft.com _..._ > On Apr 18, 2018, at 4:09 PM, Phil Kane <[hidden email]> wrote: > >> On 4/18/2018 11:28 AM, Jim Brown wrote: >> >> NEC does not apply to low voltage DC systems. > > Oh? Check NEC Section 7 , specifically Article 720, which applies > specifically to Low Voltage systems. (We do a LOT of 48V distribution > system designs.) > > Philip M. Kane Esq / P.E. (K2ASP since 1952) Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
«
Return to Elecraft
|
1 view|%1 views
| Free forum by Nabble | Edit this page |