Idiot proof equipment

[gs790]

I read the guitarnuts article on shock hazards and I have to say that this kind of information goes widely unread while it should be page one of every amplifier manual, and gigging for dummies [if such a book exists].

So this leads me to think and ask this question, although I already suspect the answer. Why is there not a en.wikipedia.org/wiki/Residual-current_device, or something of the like on the inputs of soundboards and guitar amplifiers? If someone grounds the circuit and electricity flows backward, wouldn't an RCI break that ground? What would happen is that there would be an annoying pop, and no more sound, but the guitarist/singer would be safe.

Okay let me guess. Doing so would horribly degrade the signal (your smart enough, figure it out). It's entirely impractical and too expensive (my bathroom outlet begs to differ). Finally, "It's a good idea. Someone has already thought of it and [insert technical reason here] such a circuit would not work in that application."

[newey]

gs-

Hello and Welcome!

I modified your post so as to fix your url tags, you omitted the backslash on the closing tag.

To answer your question, I believe (IOW, don't take this answer to the bank!) the reason for recommending the safety cap has to do with the brief time lag before the circuit breaker would trip out.

Whether the breaker is at the mains of your house (or at a venue), or the breaker in a residual current device (what we in the US call a "ground fault interrupter"), there will be a few milliseconds delay, not a long time, but perhaps long enough to fry certain important parts of the unlucky guitarist! The safety cap will only allow the transmission of a low voltage current for a brief moment before it fries itself, and/or the breaker trips out.

There's nothing impractical about using a GFI on your amp, but it might not provide full protection, hence the safety cap recommendation.

Also note that this sort of malfunction is very rare, only happens in tube amp circuits, and the safety cap only protects you from this while touching the strings. Other metal parts such as knobs might still present a shock hazard, since they are not wired to the safety cap.

As has been written here many times, if you are worried about the risk of this,"the best protection is no connection", that is, a wireless RF connection only.

[D2o]

Hi gs790,

I don't have anything to add to what newey has already said, but I did perform a search for "GFCI" and set the parameters (at the bottom of the search page) for the "within" time and and the number of results to 999, and found no shortage of discussion on your query.

The bottom line seems to be that there is no reason not to use a GFCI for a designated outlet, or even use one that is built in a cord, but that it is not a license to act carelessly*, and should not be considered to replace the 400V DC blocking cap. (*I'm not suggesting you are acting carelessly, by any means).

D2o

P.S. good question for a first post - welcome to GN2!

[ChrisK]

If someone grounds the circuit and electricity flows backward

Electricity doesn't have a backwards or forwards for flow direction, it just flows in all possible paths at once, limited within a given path by the impedance of said path.

An RCI detects unbalanced current flow between two conductors which comprise a single circuit loop. The unbalance indicates current leakage outside of the loop, and hence a safety fault.

Part of the difficulty in applying this to guitar amps is that the loop circuit from the guitar is minuscule and inherently unbalanced due to it being a conductor surrounded by a shield. Unbalanced current flow is not necessary a fault condition.

What is a fault condition is current sourcing out of the amplifier input back to the guitar. U.L. Listing for a product in the U.S. generally addresses safety and the effect of a single component failure causing a hazard. Unfortunately, safety standards evolve and the early standards applied to guitar amplifiers were not as comprehensive as those today.

Furthermore, many amplifiers were built by hand using materials that did not have the long-term life necessary for decades of use. Few products are produced today using rubber insulated wiring, for instance.

Also, the grounding practices (or lack thereof) of the past amplifier designs leave EVERYTHING to be desired. In the U.S. we have a three wire outlet. One is a SAFETY ground. Unfortunately, there are still many things supplied with a two wire mains cord.

Many of the instances of electrical shock from amplifiers do not come from the amplifier itself, but from the defective house and building wiring.

There are outlet testers available that determine any fault conditions and indicate them with LEDs such that anyone can determine the safety of said outlet.


You all DO use these BEFORE EVERY instance of plugging in an amplifier, whether at a performance or in your house, don't you?


The use of an RCI/GFI on every electrical outlet that might lead to an electrical shock is mandatory. Even if it's not mandated by code, it should be driven by avoidance of operator stupidity.


A young aspiring guitarist was being shocked when he played barefoot thru an old amp on a concrete floor. His uncle said "wear your shoes". I said "fix the damn outlet".


So, an RCI/GFI belongs on/in every outlet used for musical gear.


While we could design such protection for an amp, any such protection would depend on the outlet for its proper operation anyway.


The safety cap supplies a modicum of protection against erroneous AC current coming out of the amp input (capacitors pass AC as a function of their and circuit impedances), but block steady-state DC (transients still pass since they are comprised of step-functions).

The better way to practice safety cap use is to use two of them, in an insulated enclosure, at the input to the amp, capacitively coupling both the signal and shield to the guitar. Its still a band-aid.


There is that other solution that has excellent fidelity, minimal tone-suck, ease of operation, profound resistance to any electrical potential short of lightening, and has been available for years.


RF