Re: [Elecraft] Negative rail grounding

[Eric Swartz - WA6HHQ, Elecraft]

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  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)
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Re: [Elecraft] Negative rail grounding

[Phil Kane]

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
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Re: [Elecraft] Negative rail grounding

[Jim Brown]

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

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Re: [Elecraft] Negative rail grounding

[Josh Fiden]

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?


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Re: [Elecraft] Negative rail grounding

[Michael Blake]

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
k9...@mac.com


> On Apr 18, 2018, at 14:28, Jim Brown  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
> 
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Re: [Elecraft] Negative rail grounding

[Jim Brown]

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

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[Elecraft] Negative rail grounding

[Michael Blake]

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
k9...@mac.com

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