High voltage DC/DC Power Converter Interlock Safety Box

Thread Starter

SiCEngineer

Joined May 22, 2019
299
Hi all,

Although I am doing my PhD in Power Electronics and have developed a power supply capable of operating at up to 6kV, I now need to think about how to protect myself from it. I have been looking online for resources that show me step-by-step how to design and build a small box that is interlocked as such to not allow access when the HV is ON, and does not allow access until certain time has passed after HV turned OFF. Does anyone have any good references that can guide a newbie through this? Are there any other components I should look into including?
Notes:
My power supply is a DC current limited bench supply. Is there any need for a fuse, thermistor or anything as such in the box design?
Input voltage is 270VDC. The idea is to disconnect this when the door is opened, which I think is achieved with an interlock?
I believe then, I could have a button which disconnects the HV. Sets off a timer which then once elapses opens the interlock and allows the door to be opened. I think it could be quite a simple box, but I honestly do not know where to start and can only find complete systems online which I find very difficult to follow with not much experience.

I have bought a lot of HV PPE equipment which breaks the ground loop between me and the ground, but the HV Box is obviously extremely pivotal to me as well. I am not getting paid enough in my PhD to not think about how to maximise my safety before everything else! ;)

If there are any other aspects that I should include, such as pre-charge, or surge-protection? Then do let me know so I can add them to my list for research and revision.

Best regards, SiC
 

AnalogKid

Joined Aug 1, 2013
9,254
Not clear: What power sources are available continuously (before and after interlock operation) to power the monitor/interlock circuits?

To help us get up to speed, start with a truth table with all possible combinations of all input states, and the resulting output states for each.

ak
 

Thread Starter

SiCEngineer

Joined May 22, 2019
299
Not clear: What power sources are available continuously (before and after interlock operation) to power the monitor/interlock circuits?

To help us get up to speed, start with a truth table with all possible combinations of all input states, and the resulting output states for each.

ak
My input voltage is 270Vdc and the board has 15V and 5V from auxiliary supply. However I plan to buy a 25VDC AC/DC converter to power the protection relays and the like. The truth table method sounds very interesting - but I wouldn’t know where to start on that. Is there any material on using truth tables in this respect? I have a lot of experience with truth tables themselves but not in this application...
 

AnalogKid

Joined Aug 1, 2013
9,254
I have a lot of experience with truth tables themselves but not in this application...
The application doesn't matter. The method is the same whether describing a combinatorial logic system or guest seating at a dinner. If Sue is present, then Mark cannot sit next to Mary. If the left switch is open, then the 270 V must be disconnected. Etc.

The table is just a way of organizing a pile of "if these, then that" statements to make patterns apparent. I there is only one output state - 6 kV on or off - then for every combination of inputs (switches, power supply voltages, etc.) the output will be a 1 or 0. For example. 2 switches, 2 power supplies, and a cabinet door equal 5 inputs, for a 32-line table.

ak
 
I guess it depends on what sort of situation the interlock is intended to guard against. For instance, most fusible disconnects and industrial control panels have mechanically interlocked cabinet doors such that they cannot be opened while the disconnect handle is in the 'on' position unless an override is actuated. This is to guard qualified individuals against accidental shock by making it blatantly obvious that the circuit is still on.

In other cases, capacitor enclosures which are only intended to be opened for repair after installation are simply labelled with an appropriate stored energy warning label and advice to let the bleeder resistors do their job for at least 5 minutes before opening the enclosure.

If it is truly imperative that this power supply be rendered "touch-safe" within several seconds of the enclosure being opened, a sufficiently massive dump resistor would probably suffice. Only needs to have a short-time current and time rating sufficient to dissipate the capacitor charge down to 30 volts or so. Figure the shortest reasonable time someone would take between opening the door and touching an exposed live part and work from there with a margin for error.

If this is a commercial product, local, national and international codes, standards and legal precidents will apply.
 
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