HVSafety Circuit for comments

This forum is for specialized infomation important to the construction and safe operation of the high voltage electrical supplies and related circuitry needed for fusor operation.
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Daryl Morning
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HVSafety Circuit for comments

Post by Daryl Morning » Thu Nov 03, 2016 3:15 am

I tend to overthink safety [because I'm paranoid (in a plan for the worst, hope for the best way)] but I am planning a Safety Circuit in my fusor plan that would be based on my experiences with Industrial Machinery and their E-Stops expanding the idea I read here viewtopic.php?f=11&t=10361. For an initial description of the basic situation:As I have Children, pets, and a small lot, my plan is to have my fusor in a securable, encapsulated fusor "pit" to shield everything emitted from it after being measured, with a desk based Control Panel and remote control of everything input with monitoring of status and output.

The circuit would be a TBD low voltage, powered off the Keyed System Master Switch, use NC Mushroom switches so ALL the E-Stop buttons have to be up and ready to complete the circuit to allow the relays to be activated, Status LED and Visually flagging DPDT Relays every Step from the Mains Power to my Mk1 Mod0 Fusor that would be defaulted to disconnect or thru-Diode to ground to completely "safe" the system "instantly" under EMERGENCY Conditions and a microcontroller to require a specific series of protocols to initiate/re-initiate the fusor.

I would use LOTO and would use a pole and grounding line for maintenance and normal system entry as described here viewtopic.php?f=11&t=4876#p31925 but I would like to have the opinions of others who've built HV systems to get an opinion of the feasibility. If anything here would result in HV going to the control box or anything that is "do that and it's dangerous" let me know.

I suck at art and diagrams (pre-K sketches are about on par with my ability there) and don't have circuit design software but can usually explain my ideas, otherwise, I'd have a circuit diagram and construction drawings attached. I am attempting to create something to give a visual but not having much luck yet.

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Richard Hull
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Re: HVSafety Circuit for comments

Post by Richard Hull » Thu Nov 03, 2016 6:48 am

A single wired N.O. pushbutton powering up a DPDT locking relay via the N.C. mushroom emergency switch that is active on the HV supply should be all you'll need.

I use a key switch to supply power to the relay ON/emergency-stop circuit to prevent un-authorized power up.

Richard Hull
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Bob Reite
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Re: HVSafety Circuit for comments

Post by Bob Reite » Fri Nov 04, 2016 11:30 pm

Also put interlock switches on doors to the high voltage supply, or any place else where one might make contact with high voltage.
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.

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Re: HVSafety Circuit for comments

Post by prestonbarrows » Sat Nov 05, 2016 12:49 am

If you are using a commercial HV supply, it will almost certainly have a built-in interlock circuit. Check the documentation for your particular unit, but usually this will be something like a terminal which needs to be tied to ground to have any output. Any E-stops or interlocking doors are ganged up into a loop ending on that terminal. This often includes an internal dump resistor to positively ground the output when the interlock is tripped.

If you are rolling your own supply, you need to give this more consideration.

Even if you have a commercial supply with interlocking capability, it is best to have an external redundant grounding method for whenever you are not operating. Even tripping the mains lines of the supply does not guarentee safety. At these voltages, the capacitance of random parts floating out in space with respect to ground can store enough energy to kill. In the case of a fusor, the internal grid and vacuum chamber make up what amounts to a high voltage capacitor. Additionally, any usable power supply will have some built-in capacitance intentionally across the output for managing ripple. This can pose a significant hazard if not dealt with properly, especially on homemade rigs.

A silicon based switch becomes quite difficult at voltages over some 10kV. These are also not the safest because it is not obvious if a solid state switch is closed or open and they are prone to failure. Mechanical relays consisting of big bars with fail-safe solenoid actuators are common in industry. Ross Engineering http://www.rossengineeringcorp.com/ is a great catalog to drool over and get examples if you are the DIY type. These would be connected to any interlocks and 'scram' switches to immediately shut off HV in an emergency or during maintenance.

At a minimum, a grounding stick, literally a grounded metal hook on the end of an insulating rod, should be used to manually ground the output whenever you are not operating. Commercial examples of these can also be found in Ross's catalog. A series current limiting resistor is added for applications where high voltage capacitors would lead to arc flash hazards. The hook should live on the exposed HV until the second you are ready to close the final interlock barrier.

Every system I've designed uses a physical barrier such as a grounded cage or doors which must be closed to clear the interlock on the HV supply along with a mechanical grounding bar with is tied to the door interlock circuit, a manual grounding hook or both.

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Re: HVSafety Circuit for comments

Post by David Kunkle » Mon Nov 07, 2016 1:49 am

prestonbarrows wrote: Even tripping the mains lines of the supply does not guarentee safety. At these voltages, the capacitance of random parts floating out in space with respect to ground can store enough energy to kill. In the case of a fusor, the internal grid and vacuum chamber make up what amounts to a high voltage capacitor.
I can attest to that. I have a large Universal Voltronics PS with all the bells and whistles. More recently, I acquired a small, portable Universal Voltronics PS that max's at 22kV. Very limited and basic in the controls department. When I shut off the HV on this smaller unit, I noticed the voltmeter takes its sweet time going back to zero- unlike my big PS. Got suspicious and just for fun, put my HV probe on the hot lead after it was shut off. Sure enough, dang thing reads in the thousands of volts- exactly whatever the PS's voltmeter still reads. I didn't think anything is broken/fried- I figured it's just a "you get what you pay for" thing in my case. No doubt this is the same phenomenon Preston's talking about. You don't want to find out the hard way how many amps are left in there.
If your experiment needs statistics, you ought to have done a better experiment.

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Dennis P Brown
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Re: HVSafety Circuit for comments

Post by Dennis P Brown » Mon Nov 07, 2016 6:14 pm

My high voltage system (mostly home made) has all HV cable inside of it as well as all internal exposed parts are under oil inside the metal (grounded) case (access door is key locked and the key is stored separately away from the unit.) Its internal x-former is further shielded under a plastic panel. All parts inside and access doors have a HV warning signs applied. The main output cable is fully shielded and that is grounded; the 'hot' end is mated into a ceramic insulator body so no HV contact is possible when connected. My supply has two separate on switches - a master system 'On' (which is lighted when energized) and a High Voltage 'On' switch. Both must be manually turned on for the HV to exist. Aside: the master varaic must also be operated for HV. I also have two totally independent grounds to make certain neither the HV case nor the fusor can float in any manner. Finally, I check my wall outlet plug from time to time to confirm that its grounded and polarity are correct and functioning ( a low cost plug in device is available at all major Hardware stores.)

As a minor but very useful feature, I have an insulated grounding rod/cable that I use to check/insure that ANYTHING on the fusor I touch is safe even during operation. Once. for reasons that are difficult to detail (but the voltage was 'only' around 10 kV so not all metal parts were really in proper contact), a remote valve was hot (that is why I added separate grounding points to many separate areas on my fusor now!)

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Re: HVSafety Circuit for comments

Post by AllenWallace » Mon Nov 14, 2016 12:43 am

Please realized that there are safe and unsafe methods of measuring fusor current and voltage. Do have a proposed schematic to measure I and V?

We keep our fingers away from the hot stuff but yet we need instrumentation what requires us get close to with our fingers and eyes.

prestonbarrows
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Re: HVSafety Circuit for comments

Post by prestonbarrows » Sat Nov 19, 2016 5:17 am

David Kunkle wrote:
prestonbarrows wrote: Even tripping the mains lines of the supply does not guarentee safety. At these voltages, the capacitance of random parts floating out in space with respect to ground can store enough energy to kill. In the case of a fusor, the internal grid and vacuum chamber make up what amounts to a high voltage capacitor.
I can attest to that. I have a large Universal Voltronics PS with all the bells and whistles. More recently, I acquired a small, portable Universal Voltronics PS that max's at 22kV. Very limited and basic in the controls department. When I shut off the HV on this smaller unit, I noticed the voltmeter takes its sweet time going back to zero- unlike my big PS. Got suspicious and just for fun, put my HV probe on the hot lead after it was shut off. Sure enough, dang thing reads in the thousands of volts- exactly whatever the PS's voltmeter still reads. I didn't think anything is broken/fried- I figured it's just a "you get what you pay for" thing in my case. No doubt this is the same phenomenon Preston's talking about. You don't want to find out the hard way how many amps are left in there.
The inherent distributed capacitance of even a dozen feet or so of standard coax cable from spellman/glassman style supplies pushes up into the lethal limits of stored energy when you start talking about anything over a few 10's of kV. So, even if there is no explicit ripple-reducing discrete capacitance added across the HVPS output, the supply can kill you if not properly grounded.

It only takes on the order of 10's of joules electrical energy to kill.

You absolutely want a dumb 'ground-clamp-on-wire' manual fail-safe that can absolutely ground out any HV sources during manual maintenance.

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