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Author Topic: safety  (Read 21385 times)
G3RZP
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« Reply #15 on: September 13, 2013, 02:16:32 PM »

I got so paranoid about it that I don't have a wedding ring and only ever have a leather watch strap...while the XYL never wears a ring!
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K8AXW
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« Reply #16 on: September 13, 2013, 06:28:21 PM »

Peter:  I do have a wedding ring....very nice heavy gold.  However, 40 years of working in a power plant, changing brushes on the generator slip rings with the generator online, climbing ladders and on top of equipment along with the safety films of what happens when a wedding ring gets caught and you fall or step down too far.....definitely discouraged wearing it.

Then when I get home it's the work bench with both high and low voltages.....haven't had it on for 53 of my 54 year marriage.
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W9FIB
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« Reply #17 on: September 13, 2013, 06:39:10 PM »

I use a 250 ohm 25 watt resistor with battery charger type alligator clips and put it across the filter caps. It safely drains off the charge in short order...pardon the pun! I also have a LED with a load resistor in parallel across the resistor. When the LED stops glowing, you know the caps are below 2 volts. Then I can safely install a shorting wire so I know it is then safe to work on.

Anything over 100 VDC I use insulated gloves to attach the resistor wires.

Go to Youtube and look up arc flash. Prime examples of what NOT to do.
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KD0WDH
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« Reply #18 on: September 15, 2013, 07:06:34 PM »

Handle electrical circuits the same way you handle a firearm. Assume every gun is loaded, and assume every circuit is powerful enough to kill you. Use standard precautions.

73
Cheers,
Shawn
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WD3N
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« Reply #19 on: September 16, 2013, 07:20:37 AM »

Maybe SAFETY should be a sticky .
If you get electrocuted ,your XYL may give all your stuff away for free in spite .
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G3RZP
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« Reply #20 on: September 16, 2013, 08:51:05 AM »

Mine wouldn't, but she's an amateur, too.
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KF7CG
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« Reply #21 on: September 16, 2013, 11:27:37 AM »

50 volts, high or low current capability is in a dangerous range. It has sufficient voltage to possibly push just enough current through the heart to cause fibrillation without the clamping effect of higher voltages current. It is that clamping effect that is used by defibrillators.

Current through the body dangers are not straight line, so voltage dangers may not be either. By the way, always store large value capacitors shorted. They can acquire large static charges under the right conditions. Remember, lightning is static on steroids.

KF7CG
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G3RZP
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« Reply #22 on: September 16, 2013, 02:38:50 PM »

Take a 25 microfarad 30kV paper capacitor and leave it for several weeks with the terminals un-shorted. Then  CAREFULLY measure the volts across the terminals - and use a high voltage meter to do it. Or leave a electrostatic meter connected to it.

The answer is surprising......and frightening.

Back in the 1960's a LARGE (100 ton +) 275 kV transformer taken across a railroad crossing in the UK was hit by a train, with  major loss of life. But that transformer had already killed somebody by the static built up on its internal capacity. But that transformer had already killed a cleaner who unwittingly got caught by the static build up on its internal capacity......or so I was told by a guy I worked with who had worked in that department, testing the transformer..

At Marconi's in the 1960's, big HV caps were always stored with a  shorting wire across the terminals..and that wasn't for fun.

KF7CG is NOT joking..........by any means












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VU2NAN
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« Reply #23 on: September 20, 2013, 04:32:57 AM »

OK, maybe this is a stupid question.

From a safety viewpoint, which can be the most dangerous. A 3kV 1 amp supply for 1.5kW output  tube amplifier, or a 50 volt 60 amp supply for a solid state 1.5kW output  amp? ...Personally, I'm more sh*t scared with low voltage high current because years of working on high voltages have conditioned me to regard low voltages as 'safe'. But, they aren't necessarily so.

Hi OM Peter,

I suppose the danger of electric shock from a high voltage supply is greater than that of short-circuit  across a low voltage, high current one.

The dangers of short-circuiting high current battery terminals need not necessarily apply to high-current power supplies with short-circuit protection.

73,

Nandu.
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K8AXW
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« Reply #24 on: September 20, 2013, 08:45:14 AM »

Quote
The dangers of short-circuiting high current battery terminals need not necessarily apply to high-current power supplies with short-circuit protection.

Nandu:  Perhaps I don't understand your statement but my interpretation of your statement indicates that a high-current power supply with short circuit protection isn't a concern.

It should be understood that a high-current power supply the short circuit protection will not operate until the rated current is reached.  Which means that it can dump, in many cases, in excess of 25A before the circuit operates. 

Of course this all depends on skin resistance, contact surface (which is basically the same thing) and other factors.  Short circuiting by a finger ring or wrist watch can be devastating with this kind of current.

I apologize if I have misunderstood your statement.

Al - K8AXW
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M0HCN
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Posts: 473




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« Reply #25 on: September 20, 2013, 09:44:52 AM »

There is another name for a current limited heavy current supply of usually 50V or so....

...Arc welder!

The dangers are more in the way of thermal then shock, but an accident with a stiff 48V rail can still be a life altering event, and even ignoring the heavy power, a KW of RF on 2m can and will cause some unpleasantness if you get in the way of a high Z point (Been there, got the burn to prove it).

Also, for those not used to heavy DC, be careful when choosing your fusing and switching components, there is a reason a switch rated for 240V AC may only be rated for 24V DC (and at a lower switching current), same goes for circuit breakers and fuses.

Don't forget that key down, the drain on a 50V push pull stage will hit 100V even if driving into a pure resistance, and with RF you do not need to complete a classical path to ground, you have radiation resistance!

High power from whatever source is always worthy of respect.

Regards, Dan.
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G3RZP
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« Reply #26 on: September 20, 2013, 03:19:13 PM »

>High power from whatever source is always worthy of respect. <

Exactly, Dan. The problem can be that because it's low voltage, that danger isn't recognised.
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VU2NAN
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« Reply #27 on: September 20, 2013, 09:30:45 PM »

Quote
The dangers of short-circuiting high current battery terminals need not necessarily apply to high-current power supplies with short-circuit protection.

Nandu:  Perhaps I don't understand your statement but my interpretation of your statement indicates that a high-current power supply with short circuit protection isn't a concern.

It should be understood that a high-current power supply the short circuit protection will not operate until the rated current is reached.  Which means that it can dump, in many cases, in excess of 25A before the circuit operates........................................................................
.................................................................................
I apologize if I have misunderstood your statement.

Al - K8AXW

Hi OM Al,

Very true in the case of a power supply that only goes to 'current limit' on short-circuit.

But a power supply with 'overcurrent-rate-of-rise' monitoring would instantaneously trip without allowing the current to reach dangerous levels.

Your views are most welcome.

Many thanks.

73,

Nandu.


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