Connecting circuit common and safety ground

Discussion in 'General Electronics Chat' started by Veracohr, Nov 5, 2017.

  1. Veracohr

    Thread Starter Distinguished Member

    Jan 3, 2011
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    101
    I keep getting confused about when it is and isn't good to connect the circuit common to safety ground. What's the right way to do it? Are there any official standards that relate to safety?

    I'm building a musical instrument amplifier, where the part of the connectors that you grab on to for plugging things in and out are connected to circuit common. The circuit common is the center tap of the transformer secondary. It seems to me like anything that the user can touch should be connected to safety ground. On a power amplifier I have, the input and output negative leads are connected to safety ground, but on a pair of powered speakers (speaker outputs not accessible), the input negatives are NOT connected to safety ground. Also on a modular synth system I have, the connector negatives are not connected to safety ground, and it also has a linear, bipolar supply like what I'm building, albeit lower power. Another synth which uses an external SMPS does not have the connector negatives connected to either power lead.

    What should I do?
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    5,798
    Musical instrument amplifiers must be grounded. Trace all common (sleeve) on 1/4" jacks back to a COM connection on the amp. If you are using a bipolar power supply from a center tap transformer, trace all your COM back to the center tap. If this traces back correctly then connect the center tap to safety ground.

    Connections to loudspeakers are a different matter altogether. Modern power amps are usually bridged dual amps or Class-D amps. You do not connect either speak connections to ground.
     
  3. tranzz4md

    Member

    Apr 10, 2015
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    Active powered circuit conductors may not be connected to the "safety ground" at any utilization equipment or power consuming device. The grounded power circuit conductor may only connect to earth ground, along with the "safety ground", at a single point at the electrical service entrance equipment for a building or facility, or as Chips mentions, at the center tap of a power transformer supplying power to the system.

    Doing so would create a "ground fault".
     
    Last edited: Nov 5, 2017
  4. Veracohr

    Thread Starter Distinguished Member

    Jan 3, 2011
    674
    101
    Thanks MrChips. I designed my amp as a bridge type and the current-carrying speaker connections are isolated from the connector sleeve (XLR for the speaker outputs).

    I understand that the neutral lead of the power input must only connect to the ground lead at a single point at the building entrance, but your wording is strange to me and seems to contradict what MrChips said.
     
  5. tranzz4md

    Member

    Apr 10, 2015
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    Though I use the term "neutral" all the time, the correct technical term is "grounded conductor". The Britt's like "earthed", I believe. I only hear the term "safety ground" used in a working sense, kind of like a temporary or suplemental conductor. My point is that the green wire in the power cord, or exposed bare conductive parts can't have current carrying conductors connected to them.
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
    17,756
    5,403
    Where galvanic isolation occurs, such as a power transformer, effectively removing the safety earth ground and a neutral, it is permitted to designate a secondary conductor as a local neutral and connect the service earth ground to it in order to re-reference a earth ground.
    Max.
     
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