High Voltage

[martcon]

Hi All

I am new to this forum and if possible I would like to get some advice on the following please.

I attended a fault on a UV unit that has a 4-20 ma output to a logger which in turn is connected to other devices giving analogue and digital signals all of which have a common negative on breakout box of logger.

A fitter changing a reed switch on a flow meter and reported that he received a substantial shock when he was replacing the reed switch which normally works at 3.9 v DC. I measured voltage on pair of wires to reed switch and recorded 3.9V DC but I did notice that my volt tick was showing high AC Voltage present, I then measured from each leg of wire to reed switch and recorded 125 Volts AC to main earth and neutral bar on distribution board.

I eventually traced the 125 V AC to 4-20 ma output on UV unit and found it had back fed via logger breakout box(common negatives) and in turn was present at all devices connected to logger. I thought at first this might be a floating/induced voltage but while plugging my laptop into military comms port on logger I received a pretty strong shock myself(I Know!Lol). I connected a 110 volt panel light to one leg of 4-20ma output from UV unit and a neutral from dist board to other side of panel light and it lit taking full rated load(very small) but this indicates(to me???) that this voltage is capable of generating a load and in turn could have the potential to injure somebody.

The UV unit is supplied by a 220 Volt AC supply and the manufacturers are saying that having 125 Volts AC on this output is impossible as the card is fed by a 3.9VDC power supply.

Could any of you give me some advice on this or a reason why/where the 125VAC is coming from or if it has the potential to be lethal. Measuring across the 4-20 ma output on UV unit gives 14V Dc but either leg of it to earth or neutral gives 125 V AC.

Thanks in advance.

 

[junkhound]

Sounds like somebody added a 0.2uF (or smaller, hopefully not larger) cap to chassis for emi noise reduction or something similar. Could be there are additions someone made to the breakout box that mfg does not know about.

Biggest risk is 'startle shock' causing someone to jerk and drop something of fall off a ladder (if any situation like that possible).

See the 'how dangerous 14V...' post for some data.

Years ago in the USA, there were tube radios using what was called the "all american 5"*, meaning they could run on the antiquated (even then) 110V DC distribution still existing in some parts of NY city or off 115 Vac common in the rest of the country. Many of these also included a phono player. To reduce 120Hz HUM from the ripple residual on the plate supply, there was often a 0.2 uF cap added to chassis, which reduced the hum but also let a 60Hz leakage current of 10 mA flow to someone touching the chassis (e.g. the phono tone arm) whilst also otherwise grounded and 50% chance depending on 'polarity' of cord plug connection (no grond pin or wide ground blade in those days, circa 1950.

Yep, my first electrical shock experiences, esp if were were sitting on the grounded cold air register in the house<G>

* so called as the set would run off ac or dc, the series sum of the 5 tube's filament voltages added up to around 115V, IIR, a 50C5, rectifier was the big drop, then 12AV6, 12BA6, 12BE6, 35W4.

 

[Jraef]

Pursuant to junkyard's post, it's also likely that there is a bad neutral bond somewhere in the power system, and that some chassis ground in the electronics hooked up through a cap is trying to become that neutral bond for the entire installation.

 

[ggunn]

Sounds like somebody added a 0.2uF (or smaller, hopefully not larger) cap to chassis for emi noise reduction or something similar. Could be there are additions someone made to the breakout box that mfg does not know about.

Biggest risk is 'startle shock' causing someone to jerk and drop something of fall off a ladder (if any situation like that possible).

See the 'how dangerous 14V...' post for some data.

Years ago in the USA, there were tube radios using what was called the "all american 5"*, meaning they could run on the antiquated (even then) 110V DC distribution still existing in some parts of NY city or off 115 Vac common in the rest of the country. Many of these also included a phono player. To reduce 120Hz HUM from the ripple residual on the plate supply, there was often a 0.2 uF cap added to chassis, which reduced the hum but also let a 60Hz leakage current of 10 mA flow to someone touching the chassis (e.g. the phono tone arm) whilst also otherwise grounded and 50% chance depending on 'polarity' of cord plug connection (no grond pin or wide ground blade in those days, circa 1950.

Old Fender guitar and bass amplifiers used to be set up that way. Replacing the two conductor power cord and "suicide capacitor" with a grounded power cord on these amps is a modification acceptable to even the most hard core purist vintage amp collectors if they ever plan to use the amp.