Elements between PCB-GND and Mains Earth in SolidState Power Amps - How to Calculate? - Page 4 - diyAudio
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Old 25th June 2012, 04:43 AM   #31
jaycee is offline jaycee  United Kingdom
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Note that I said use a 10 ohm 5W resistor. For a 230V supply, this will allow 23A to pass. The resistor should survive long enough for a fuse to rupture or a breaker to trip. If you want to be sure, a 10R NTC would be even better as it would decrease in resistance the longer the current flows.

The ground terminal of the power connector should always go direct to chassis. The ground loop breaker should be connected between the chassis ground point and the PSU 0V star.
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Old 25th June 2012, 07:01 AM   #32
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"Note that I said use a 10 ohm 5W resistor. For a 230V supply, this will allow 23A to pass. The resistor should survive long enough for a fuse to rupture or a breaker to trip. If you want to be sure, a 10R NTC would be even better as it would decrease in resistance the longer the current flows."

I think this is very poor practice.

The connection from the chassis safety ground to the supply ground star is to lower noise, no real current should ever flow through it. A pair of inverse-parallel connected diodes bypassed by a parallel resistor and parallel cap offer the best noise performance and safety. If one of the other pieces connected to the unit in question has a ground problem it will buzz like heck, letting you know that you have a problem to run down.

The diodes are best be ones rated at 400A surge, and ordinary (not fast type) meet this requirement. If the fuse/circuit-breaker is not fast enough on a fault, the diode will fail as a short, so you still have protection.

23A will cause the 10Ω/5W resistor to dissipate about 5.3KW, and possibly burst into flames and maybe produce shrapnel.
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Old 25th June 2012, 07:59 AM   #33
jaycee is offline jaycee  United Kingdom
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In truth 23A would not flow, though. The primary side fuse is going to be what, 2A at the most in a domestic amp ? Even a time-delay type would blow quickly.

Agreed, if this were a PA amp where you might have a 1000VA transformer or so with a large primary fuse, and likely not running from a circuit protected by an RCD, I would use the diodes.

In the UK we have extra protection as our wall plugs have their own fuses. A typical IEC cable is protected by a 13A fuse in the plug. A primary-secondary short with a 10 ohm resistor would blow this very quickly.
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Old 25th June 2012, 08:50 AM   #34
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Most fuses will stand 200% of rated current for two minutes, so that 13A fuse may not blow as fast as you think. You are also assuming a short to ground, and near the top of the winding, none of which should be assumed.

Fast-Acting Ceramic Tube Fuses

http://www.cooperindustries.com/cont...C-V_Series.pdf
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Last edited by djk; 25th June 2012 at 08:54 AM.
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Old 25th June 2012, 09:17 AM   #35
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I want to know more exactly informations arround this topic. E. g. the fact, from where comes the high current flow (i. e. the potential difference between the mains earth and the audio signal GND) under certain conditions through the so called "Disconnected Network" in usual domestic environments.
By several service work I must replace such devices, but after replace this parts, it's never happened that they blown away once again.
Thus it is impossible for me to determine an exact cause.

Therefore the same question as in one of a previous post from me:

What is the most usual english term for "Disconnecting Network" ??
The provided results from Google by this term is too low.

Last edited by tiefbassuebertr; 25th June 2012 at 09:21 AM.
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Old 25th June 2012, 09:45 AM   #36
AndrewT is offline AndrewT  Scotland
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Just as a hypothetical worst case example, suppose the Live wire feeding the transformer pops out of it's screw down terminal block, or the stranded wire just breaks.
The loose end springs away and briefly flicks past an audio connection, blowing up the audio stage and finally lands on the Main Audio Ground wire attached to the speaker terminal.
Many of the audio side components are burnt out, some have turned into a carbon mess. All the audio grounds are at LIVE voltage and the mains fuse has not blown yet.

The mains wiring impedance from Distribution board to amplifier and back again is ~0.2ohms.
The chassis impedance from LIVE to Earth is ~0.01ohms.
The voltage applied across these is 240Vac. A current of ~1142Aac starts to pass from LIVE to PE. The Mains fuse blows in <1ms.
The operator/user is exposed to danger for just that 1ms - IF THEY HAPPEN TO BE TOUCHING THE FAULTY APPLIANCE.
But they are not exposed to the full 240Vac. The faulty appliance is located roughly half way around the distribution route from LIVE to Neutral at the distribution board. At the very worst the appliance will rise to ~120Vac for that <1ms till the fuse blows. In the UK they is a further protective rule : the faulty appliance and the distribution wiring must limit the voltage on the LIVE appliance during the fault incident. I think this limited voltage is 50Vac "by design"
The design of the house wiring installation and the handover test must both show that the limiting voltage will not be exceeded.

Now, let's put a 10r resistor in that fault current return route.
The fuse could now take many hundreds of milliseconds to blow, or in some worst combination of cases even many seconds to blow.
The user is now exposed to a higher voltage and for much longer.

Now the really bad situation:- the potentially fatal version of events.
The fault occurs, the 10r resistor burns out while trying to dissipate those many kW of power. The fuse does not blow !!!! The voltage on the audio Ground is now 240Vac. The music system sounds quiet. The user gets up from his/her seat and fiddles with a control knob or checks the speaker cable or "POOF" they are gone !!!!!!!
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Last edited by AndrewT; 25th June 2012 at 09:51 AM.
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Old 25th June 2012, 09:52 AM   #37
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by djk View Post
Most fuses will stand 200% of rated current for two minutes,
A T rated fuse is likely to be even slower to rupture.
Then you have the production tolerances to add on. That two minutes can become 30minutes if all the worst case situations are added together.
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Old 25th June 2012, 11:07 AM   #38
djk is offline djk
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A slow-blow type may pass 200% for one hour, much more over shorter periods of time. I used to use a pair of TL30 to fuse a distro, it could pass 120A of lights and sound for a live stage-show (one hour sets).

Guitar amps with hum switches (line phase-reversal) are a real problem, I've had them send 120V down a snake, through the FOH gear, back up the snake, and blow the input opamp in the electronic crossover. Designing a line level piece to handle 120V on the input is a chore.
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Old 2nd July 2012, 12:10 PM   #39
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Quote:
Originally Posted by jaycee View Post
Note that I said use a 10 ohm 5W resistor. For a 230V supply, this will allow 23A to pass. The resistor should survive long enough for a fuse to rupture or a breaker to trip. If you want to be sure, a 10R NTC would be even better as it would decrease in resistance the longer the current flows.

The ground terminal of the power connector should always go direct to chassis. The ground loop breaker should be connected between the chassis ground point and the PSU 0V star.
have a look to that 10 ohm resistor in the Cyrus ONE - go to the second picture from post #31 about
Cyrus One
search for R121 (in front of the black potentiometer shaft).
It seems to be only a 0,125W resistor.
There are no additional components like diodes or caps on the solder side of the main PCB.
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Old 2nd July 2012, 05:45 PM   #40
djk is offline djk
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Hafler (and many others) have used resistors (without the diodes) in the range of 0R5~10R/1/4W in this position. They burn up instantly in the event of a fault.

Rockford Fosgate (parent company of Hafler) ceased using the 10R/1/4W in this position as they burned up so frequently, the now malfunctioning amplifier needing a warranty repair. As a consequence of replacing this resistor with a wire link the ground trace in the device driving the amplifier now vaporizes from carrying the fault current down the shield. This moved the repair cost from the amplifier to the driving device.
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