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Author Topic: Testing MOV surge protectors Joules of energy absorption  (Read 1787 times)

Mike Sokol

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Re: Testing MOV surge protectors Joules of energy absorption
« Reply #20 on: April 18, 2018, 01:16:45 pm »


Would it be a better test (or multiple tests) using one value till fail, then 2x value till fail, 4x... until shock equipment is maxed out or the MOV fails on a single hit, then compare whether total number of joules absorbed are the same (or similar)?

frank

Methinks I need to do both types of tests. Of course, doing 1/10 of the 5,000 rated joules test (500 joules) at least 10 times would be important. But they also want to know if their product can absorb a single 5,000 joule spike without vaporizing. The first multi-test will confirm that the MOVs are sufficiently robust, and the second overload test will confirm that the wiring can withstand the peak current without vaporizing. And yes, I need a high-speed camera and such. I'll be talking to their engineers about this in a few weeks, so I just need as many ideas swimming in my head for the initial consult. Then it will be down to budget and timing.
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John Roberts {JR}

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Re: Testing MOV surge protectors Joules of energy absorption
« Reply #21 on: April 18, 2018, 01:45:46 pm »

I could tell you everything I know about MOV testing in a very short post... I'm finished.  8)

Can I ASSume they are buying somebody else's off the shelf MOV? How about asking the MOV manufacturer for advice?

A couple decades ago I had a difficult problem troubleshooting behavior of a triac based power switch. In that case the triac manufacturer was zero help. The difficulty may be reaching the person or people at the firm who know, to ask.

JR

PS: A friend who manufactured a high voltage capacitor discharge strobe flash for photography, built a protective thick plastic(?) shield for manufacturing QA to deal with exploding capacitors. 
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Jonathan Johnson

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Re: Testing MOV surge protectors Joules of energy absorption
« Reply #22 on: April 18, 2018, 02:57:43 pm »

Methinks I need to do both types of tests. Of course, doing 1/10 of the 5,000 rated joules test (500 joules) at least 10 times would be important. But they also want to know if their product can absorb a single 5,000 joule spike without vaporizing. The first multi-test will confirm that the MOVs are sufficiently robust, and the second overload test will confirm that the wiring can withstand the peak current without vaporizing. And yes, I need a high-speed camera and such. I'll be talking to their engineers about this in a few weeks, so I just need as many ideas swimming in my head for the initial consult. Then it will be down to budget and timing.

So it's not only determining the durability of the MOV, it's also about determining the failure modes under various circumstances.

Knowing the failure modes is important, so the manufacturer can know how to best contain a failure and ensure the devices protected by the MOV remain protected in the event the MOV fails.

There are a lot of (cheap) surge protectors out there with a "protected" LED that's just connected across line and neutral -- it doesn't actually tell you that the MOVs are working. And if the "protected" LED is being truthful, it still doesn't tell you that the MOV has only one joule of protection left... and when the MOV does fail, there's nothing in there to break the circuit.
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Mike Sokol

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Re: Testing MOV surge protectors Joules of energy absorption
« Reply #23 on: April 18, 2018, 03:56:15 pm »

So it's not only determining the durability of the MOV, it's also about determining the failure modes under various circumstances.

I think so. This is a new surge protector that will be built by an OEM supplier to the market. And yes, my first plan is the contact the MOV manufacturer once I know who it is, and talk to the engineer designing this product, etc...

Part of the design issue is likely that just because a single MOV behaves a certain way in a circuit, how do you know how a ten of them will react. The peak shunting currents would go up by a factor of 10. Heating on the circuit would go up by 10 times the watt energy, other weird things I don't know about could occur. For instance, just how closely matched are these MOV's. You just can't slap 10 transistors in parallel and expect them to share the load equally.

The RV industry is in a period of consideration that all RVs should have some sort of surge protection, likely because of me writing so much about it. But I'm really promoting the idea of everyone getting an "intelligent" surge protector with a relay that can disconnect your RV from a power source that's too high, too low, or has lost its ground. Gonna be interesting to figure out exactly what's happening. I'll be under an NDA so I would be able to post the nitty gritty here. But at least I can post the scientific parts. 
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Mike Sokol

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Re: Testing MOV surge protectors Joules of energy absorption
« Reply #24 on: April 20, 2018, 10:51:51 am »

Just doing a little armchair engineering. I can get a 16 piece lot of capacitors that are 450-volt / 5600 uF. I can stack them 8 high for 3,600 volts, which also divides the individual capacitance by a factor of 8 for 700 uF per stack. Now I run the two stacks in parallel for a big capacitor of 1,400 uF at 3,600 volts. According to my quick Joule calculator at https://www.electronics2000.co.uk/calc/capacitor-charge-calculator.php that works out to just over 9,000 joules of storage. Am I thinking about this correctly or is it the French wine talking?
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John Roberts {JR}

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Re: Testing MOV surge protectors Joules of energy absorption
« Reply #25 on: April 20, 2018, 11:44:53 am »

When stacking capacitors in series you need to provide some strategy to force them to share the voltage equally. The common technique is parallel resistors forming a voltage divider. Without a resistor string higher and/or lower leakage in individual caps will create over voltage across non leaking caps. One shorted capacitor could cause overvoltage in all the other series capacitors. High voltage power supply needs to be current limited to protect against such faults.

 I would be nervous (careful) when working around this much energy potential. Probably worthwhile budgeting a robust resistor divider string power budget. The good news is this will discharge the caps when power is removed. 

JR

PS: Perhaps active devices would provide the parallel divider string using less power consumption but this ups the complexity/cost.
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Stephen Swaffer

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Re: Testing MOV surge protectors Joules of energy absorption
« Reply #26 on: April 20, 2018, 12:55:29 pm »

I spent yesterday studying arc flash as it relates to electricians.  I'm not going to pretend to know how to calculate an arc flash incident rating from the information given-but if I am doing quick calculations correctly, you could be looking at arc energy equivalent to a short at 3.6 kV, protected by a 75 amp fuse, or 480 volts protected by a 600 amp fuse (based on the device opening the circuit in 2 cycles).  From experience using software to do some arc flash calculations, I am guessing this would put you into at least a 2 rating-requiring FR clothing at appropriate incident ratings, faceshield, etc. Most electrical installs have transformer impedance as well as wire run impedance to limit short circuit current. I have seen bus plug covers bent from that kind of energy-you have both a thermal and concussive hazard.  I could be being overly cautious-but better to consider this before you build it.
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Mike Sokol

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Re: Testing MOV surge protectors Joules of energy absorption
« Reply #27 on: April 20, 2018, 01:21:25 pm »

When stacking capacitors in series you need to provide some strategy to force them to share the voltage equally. The common technique is parallel resistors forming a voltage divider. Without a resistor string higher and/or lower leakage in individual caps will create over voltage across non leaking caps. One shorted capacitor could cause overvoltage in all the other series capacitors. High voltage power supply needs to be current limited to protect against such faults.

 I would be nervous (careful) when working around this much energy potential. Probably worthwhile budgeting a robust resistor divider string power budget. The good news is this will discharge the caps when power is removed. 

JR

PS: Perhaps active devices would provide the parallel divider string using less power consumption but this ups the complexity/cost.

I built this exact same storage bank about 40 years ago for a 2K Joule (2,000 watt-second) surplus airport landing strobe. I stacked enough capacitors to get up to the 3,000 volts or so I was producing from a commercial microwave transformer, and used a car ignition coil to initiate the strike on the strobe lamp. The lamp itself was HUGE, with an outer Pyrex shield the size of a soda can, and the coiled lamp inside was finger size IIRC. It drew a full 20-amp from the 120-volt line for at least 2 seconds to reach full charge, and when we flashed the thing it could completely overwhelm 48KW of conventional tungsten PAR cans. Good times in the mid-70's.

Yes I know this is really dangerous to build, and really dangerous to use. But if the company has a serious enough budget I'll make it really safe with a remote control outside of the blast zone. This is serious mojo and not to be taken lightly. I can't believe I built and tested my 2KWS strobe on an open bench without killing myself. And it was really dangerous to set off as well since it was as bright as an arc welder. So don't worry, I'll be super safe and triple-check my work. This thing will have as much stored energy as a small bomb and I don't take it lightly.

But it is interesting, isin't it?
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Jonathan Johnson

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Re: Testing MOV surge protectors Joules of energy absorption
« Reply #28 on: April 20, 2018, 04:45:28 pm »

Yes I know this is really dangerous to build, and really dangerous to use. But if the company has a serious enough budget I'll make it really safe with a remote control outside of the blast zone. This is serious mojo and not to be taken lightly. I can't believe I built and tested my 2KWS strobe on an open bench without killing myself. And it was really dangerous to set off as well since it was as bright as an arc welder. So don't worry, I'll be super safe and triple-check my work. This thing will have as much stored energy as a small bomb and I don't take it lightly.

But it is interesting, isin't it?

Maybe the folks at the Bonneville Power Administration's high voltage lab (located in Vancouver, Washington) could offer some advice. They've got a lot of experience discharging huge amounts of stored energy safely.

http://www.today.com/video/today/55754864

http://www.columbian.com/news/2013/dec/09/high-voltage-bpa-lab-a-rare-asset-ross-complex/

https://www.bpa.gov/
« Last Edit: April 20, 2018, 04:54:33 pm by Jonathan Johnson »
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Mike Sokol

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Re: Testing MOV surge protectors Joules of energy absorption
« Reply #29 on: April 23, 2018, 06:56:11 am »

Maybe the folks at the Bonneville Power Administration's high voltage lab (located in Vancouver, Washington) could offer some advice. They've got a lot of experience discharging huge amounts of stored energy safely.

http://www.today.com/video/today/55754864

http://www.columbian.com/news/2013/dec/09/high-voltage-bpa-lab-a-rare-asset-ross-complex/

https://www.bpa.gov/

That's a great idea. I've talked to one of their engineers two years ago about induced voltage from overhead power lines, and they were very helpful. I'll give them a call about this project and see what kind of research they've done already.
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Mike Sokol
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