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Hot chassis circuit made safe?
Hot chassis circuit made safe?
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Old 22nd March 2019, 02:04 PM   #21
hex69 is offline hex69  Canada
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Quote:
Originally Posted by Gnobuddy View Post
The thing is, damp / wet areas are exactly where lethal shocks are most likely to happen. I think having a GFCI to protect musicians playing in such locations would be a very good idea, so much so, that it really should be mandatory.
I know and have nothing against some more protection, I was just wondering if the usual GFCI used in bathroom were designed to hold a tube amp without tripping needlessly, the industrial one PRR posted seems more suitable, for sure it's a great concept I'm with you on this, please share the result as soon as you did that on your amp.

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Originally Posted by Gnobuddy View Post
DIY is what I was thinking too. We DIY types might think it's worth shelling out an extra $20 or $30 to avoid electrocution. But Big Musical Corp. Inc would loose far too much annual profit if they increase the raw cost of each amplifier by ten bucks, so they would never consider it unless it became law and they were forced to comply.
Yes that's the big fun in DIY, I'm not challenging you about the concept at all, but I wonder if at least one equiped GFCI amplifier has been released on the market. I agree that it should be mandatory, not in amplifiers and gears but in all houses just like they do in Danish and probably other places.

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Originally Posted by Gnobuddy View Post
I'm not sure how common it is for GFCIs to trip by mistake. My wife uses her hair-dryer in the washroom, plugged into a GFCI outlet by the sink. I've never heard her complain that the GFCI shut off, even once in all the years we've been married.
Sometimes the outlets in bathroom are a little isolation transformer.. That's how it is where I live, if I remember they are rated at 20VA.
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Old 22nd March 2019, 02:06 PM   #22
hex69 is offline hex69  Canada
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Quote:
Originally Posted by leadbelly View Post
GFCI rarely used for rigs, but it IS a good idea. I don't think I would put one inside an amp, but I would include one in any DIY power strip, such as a DIY pedalboard or DIY rack.
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Old 22nd March 2019, 02:33 PM   #23
hex69 is offline hex69  Canada
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The OP asked about how to make hot chassis safe, I think it's a good idea to have such tester then make sure to check the outlet wiring is correct, those are warning you when the ground is missing or when the hot neutral is reversed, I always have one of these on my bench, another one inside my guitar case. These are for north america but I'm sure every places have something similar.

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Old 22nd March 2019, 05:01 PM   #24
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by hex69 View Post
...I'm not challenging you about the concept at all...
Oh, feel free to challenge me, it's something I know very little about. Electronics I'm comfy with. Electrical wiring, especially high voltage, high power stuff - that I know very little about.

And it is an entirely different world, with unimaginably different hazards. For example, a little household dust stirred up by air movement when you open the electrical panel access cover can cause an arc-flash that can burn your face off in a fraction of a second.
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Originally Posted by hex69 View Post
Sometimes the outlets in bathroom are a little isolation transformer.. That's how it is where I live
I have no electrical certification (though my wife thinks I'm certifiable ), but as far as I know, electrical code in California (where I used to live), and British Columbia (where I live now), both require GFCI outlets if they are within six feet of a sink or bathtub. Here's some info for BC: https://ringelectric.ca/where-are-gfis-required/

When I was looking for information on GFCI use recently, I found out they are also required for hot-tub installations: https://www.poolandhottubdepot.com/new-hot-tub-set-up/

I've never seen the bathroom isolation transformer approach you described. Perhaps it is an older system?

I've also never heard of the whole-house system that Rasped mentioned is used in Denmark. I wonder if it is one device for the whole house, or perhaps every circuit-breaker on the main panel is of the GFCI type?


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Old 22nd March 2019, 05:38 PM   #25
aut0m4tic is offline aut0m4tic  United States
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I'm not an electrician, I'm a lineman (power poles), so my $0.02

The gfci monitors the difference between phase and neutral currents in your mains outlet. At a roughly 30mA difference, the gfci opens the phase, which disconnects power from whatever its protecting.
This will protect you during a fault in primary side of your amp. (120v in US, 240v in other parts of the world)
A gfci may not protect you on the secondary side of the transformer (which your guitar is connected to) as a phase to chassis fault on the secondary can produce equal currents on the primary side, and not trip the gfci.
It all depends on if your mains neutral is connected to the chassis or not.

The OP question about making one safe, is easy. Add an isolation transformer, and properly ground the amp. Feeding the whole thing through a gfci will add that last line of defence if something goes wrong with the mains.

At a gig, take a voltmeter and check for potential differences between your guitars and mics. Especially if using multiple power sources and pieces of equipment.

We always assume everything in the building is wired correctly, but know its usually done by the lowest bidder.

Cory
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Old 22nd March 2019, 05:56 PM   #26
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by aut0m4tic View Post
A gfci may not protect you on the secondary side of the transformer (which your guitar is connected to) as a phase to chassis fault on the secondary can produce equal currents on the primary side, and not trip the gfci.
Now that is very relevant indeed. Thank you very much!

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Originally Posted by aut0m4tic View Post
It all depends on if your mains neutral is connected to the chassis or not.
Not. Otherwise there's the risk of the entire chassis becoming live if the outlet is miswired and live and neutral are cross-connected, no? That was exactly the problem with those horrifying old "hot chassis" guitar amps.

So AFAIK the mains live and neutral are kept completely isolated from the guitar amp chassis, and only the mains earth wire is connected to it. That's how I wire my DIY guitar amps.

Which, if I understood you correctly, means a GFCI might not add any additional safety if a fault occurs on the secondary side of a guitar amps power transformer.


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Old 22nd March 2019, 10:06 PM   #27
leadbelly is offline leadbelly  Canada
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Not sure where the 30mA is from. For a 15A GFCI, it's 5mA +/- 1mA for UL & CSA certification.
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Old 22nd March 2019, 10:31 PM   #28
Gnobuddy is offline Gnobuddy  Canada
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30 mA would be pretty bad. The attached data is from Ohio State University ( Electrical Safety: The Fatal Current ). I highlighted the relevant bit.


-Gnobuddy
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File Type: png shock_vs_current.png (70.9 KB, 81 views)
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Old 23rd March 2019, 12:24 AM   #29
PRR is online now PRR  United States
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> I'm stunned that not a single guitar amp manufacturer so far seems to offer a built-in 9V DC power jack for your FX pedals.

One did (does?). Did not catch on. TrueTone?

> called HPFI in Denmark. My entire house is protected by one of those.

And RCB in the UK; and they too sometimes do it whole-house. In more casual US wiring, GFI has grown from per-outlet to per-circuit, but whole-house seems dubious to me. How do you find the problem and reset the breaker if ALL the lights are out? But whatever works.

> I'm not sure how common it is for GFCIs to trip by mistake.

It's a common complaint. Especially when a specific technology (there have been several) is new. Also cascaded GFIs seem glitchy.

> Sometimes the outlets in bathroom are a little isolation transformer.. That's how it is where I live

That is different. Common in the UK, unknown in the US. Dedicated low-voltage supplies for lighting and shaver.

> I think it's a good idea to have such tester then make sure to check the outlet wiring is correct, those are warning you when the ground is missing or when the hot neutral is reversed

Yes; but there are at least two faults these will NOT reveal.

> a little household dust stirred up by air movement when you open the electrical panel access cover can cause an arc-flash that can burn your face off

Never heard that one. Seems unlikely on 120V or 240V lines. Even in quite dusty fuseboxes.

However opening a strange fusebox IS dangerous. The hazards I have seen! Sometimes ready to fall out in your face. If you look on Inspection Horrors sites you can find breakers and even buses and feeders over-stuffed or rusted-through and ready to bite.

> We always assume everything in the building is wired correctly, but know its usually done by the lowest bidder.

Or by slipping a case of beer to some guy who once spent a day as Electrician's go-fer. I'm quite sure much of the framing and wiring in this house was done by beer-buddies.

> electrical code in California (where I used to live), and British Columbia (where I live now), both require GFCI outlets if they are within six feet of a sink or bathtub.
> When I was looking for information on GFCI use recently, I found out they are also required for hot-tub installations


GFI around water has been in the NEC for a long long time, and is pretty universally enforced (in new or major refurb) by local code officers.

Hot tub GFI and a LOT of grounding rules was a hot topic in NEC at the end of the 20th century. There were several revisions and clarifications. If you are wiring a hot tub you want a quite recent copy of NEC. (I have six, from 1919 to 2009.)

> Not sure where the 30mA is from. For a 15A GFCI, it's 5mA

Look at the spec. It is a *Time Curve*. 30mA will trip quickly. 5mA will trip after some time. I strongly suspect that UK-spec "30mA" and US-spec "5mA" are the SAME calibration, but the nominal names taken from different points on the same curve.
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Old 23rd March 2019, 12:38 AM   #30
Tubelab_com is online now Tubelab_com  United States
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Quote:
I was just wondering if the usual GFCI used in bathroom were designed to hold a tube amp without tripping needlessly,
My lab is in a concrete basement which could be damp by definition and therefore requires protection by GFCI. Every outlet in the basement is wired to a single 20 amp GFCI breaker, and my workbench has its own 30 amp GFCI. I have been experimenting on all sorts of electronics and nothing I have done has tripped the breaker. These were breakers installed by the builder, and likely the cheapest stuff available. The treadmill however randomly trips the GFCI whenever it pleases. I haven't figured out why either.

Quote:
The gfci monitors the difference between phase and neutral currents in your mains outlet.....This will protect you during a fault in primary side of your amp. A gfci may not protect you on the secondary side of the transformer.
This is true, however the biggest risk in a guitar amp, a musical instrument, or any DIY device, is a fault connecting live (primary side) mains voltage into any circuit accessible by the user. It is usually the case that something, the PA, mixing console, or other large part of a sound reinforcement system is grounded through the mains ground connection. This should put mics, synths and most of the on stage equipment at ground level electrically. A guitar amp with a shorted death cap could put the guitar at line voltage potential, such that touching both could be disastrous. Stage lighting "cans" especially if they have ever been wet, are notorious "shockers." A properly installed and wired GFCI will trip in this case possibly saving someone's life.

A short somewhere inside the amp connecting B+ or some other high voltage on the secondary side of the power transformer to the input can not be seen or detected by the GFCI, however such a short SHOULD BLOW the amps fuse since all user inputs SHOULD be connected to circuit ground.

It has been debated in the HiFi world as to whether it is necessary to ground one side of the speaker winding on the OPT. I have seen an OPT with a primary to secondary SHORT, and it was in a guitar amp. The amp operated normally with no clue that there was 300 volts on the speaker leads until someone tried to attach a DI box and stuff blew up. We SHOULD GROUND one side of the speaker for this reason.

A GFCI will not protect you from any acts of stupidity on the secondary side of a power transformer. This includes touching the case of your TO-3 output transistors with one hand to see if they are hot while holding the guitar in the other hand.......especially when doing an outdoor gig in the Florida sun with a live mic nearby so that the audience all gets to hear the "F" word!.....No, it wasn't me.

The manner in which we grip the neck of the guitar will make it nearly impossible to let go of it if a AC current fault through the player occurs.
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