Capacitor Dividers

Is this a quiz?

https://www.jameco.com/Jameco/workshop/Video/incorrectly-using-capacitors.html

Hi Guru,

Thank you for forwarding the thread.

This is not a quiz but just trying to figure out whether the arrangement below can be used as a live line detector. Please note that the applied voltage and value of the voltage dropping capacitor have changed (15 KV instead of 3.3 KV and 150 pF instead of 600 pF)

Assume that the 150 pF capacitor is it its completely discharged state when the voltage is applied and at peak voltage (21.21 KV). Load will be of high impedance.

Regards;

I still stand by my comment that you should not be working anywhere near kilovolts of electric power. Since you've performed some of the math I'll point out some of the problems with your circuit analysis.

Your analysis relies on ideal components that have no parasitic paths. Real parts have parasitic paths. Your high voltage divider component has a 17.7 Meg ohm reactive impedance. For parasitic paths to be considered trivial the rule of thumb value is for all these paths to sum in parallel to be greater than ten times the planned path. In other words, the parasitic path must be greater than 177 Meg ohm reactive in all temperatures and humidities for this ratio to be reasonably accurate for all circuit paths around this capacitor. This is possible but not a trivial task. In addition there can be electrostatic and magnetic coupling effects that can add or subtract to the final measured result. Safely designing, building and integrating a proper high voltage attenuator is a task for professionals.

Lastly but certainly the trump card here, multiple kilovolt power distribution is unforgiving. The protection circuitry designed into the grid is intended to minimize damage to the grid, not your attenuator, your detection circuitry nor any unauthorized fool touching it. An arc flash will kill.

This was just a 480V arc flash. Leave this to the professionals trained to service the grid!

First, Guru is not a name.

Second, you still didn't say what the voltage rating of the 150 pF capacitor is.

When I was in high school, my friend and I did some "experiments" with capacitors. We were pretty bright young boys and we thought it would be fun to "over stress" different caps. We took a zip cord and attached the cap. Then we laid it on the work table and plugged the other end in!

The first was a disc capacitor. I don't remember it doing anything - boo!

The second was an electrolytic. That was a fun one - blew up like the ones in the video you reference.

The third was a mylar cap. We thought it would blow up good, but it didn't - major disappointment.

The fourth was a small silver 300V cap (looked like a cylindrical resistor. No explosion - what's going on. As we detached it from the wires - zzzzaaap! The cap had enough charge, so when I touched the two leads, I got a shock.

Brings back old memories!

100%

The failure mode will begin with swelling and leaking of the capacitor,followed by an explosion.All three stages can happen simultaneously. You realize of course,that when capacitors are connected in series,the total capacitance will be less than the smallest capacitor in the series,don't you? Like resistors in parallel.

"You realize of course,that when capacitors are connected in series,the total capacitance will be less than the smallest capacitor in the series,don't you?"

True, but the voltage will be higher.

I presumed (silly me) that anyone familiar with series capacitors would know this.If not,they should not be experimenting with them,esp. at the stated voltages.

Don't make that assumption! The OP may or may not have experience. And you're right, 15KV is very, very dangerous.

I have no idea because you do not provide enough information to provide a failure prediction.

Since you do not understand what is relevant to make such a failure prediction I can say one thing with absolute certainty. STOP! You should not be working on 3300V power at all.

I second it.

Funny, he never said if the 3300 V line was AC or DC. If one assumes he meant AC because of "(Line to ground)" then its one kind of divider. If on the other hand it was a DC voltage, the situation is a little different, but the result is the same.

From what he is asking, it looks like he is hoping the higher rated voltage capacitor was somehow going to protect the lower rated voltage capacitor from exceeding its rating. Ha! This sounds like a quiz question where current flow may allow you to get away with it once or twice. But bounce the connection switch and all bets are off. Oh, by the way, where does one find a reasonable connection switch rated at 3300 volts? Not at my hardware store.....

They also never said what voltage rating will the 600 pFd capacitor have. Then there is the question of what if any other circuitry (terminal blocks, wiring, voltage sources) will be connected to these two, ill-defined components. A greatly current limited voltage source like a Megger or Tazer could probably be connected in series with these capacitors forever without serious damage to either capacitor.

Hi Redfred,

Regards,

Hi Ordinary Joe,

I wonder in what country 3300 volt DC LINE being used and I do not mean to be funny. Pardon my ignorance; since capacitors block DC a practical DC voltage divider cannot be built using capacitors. You can use capacitor only in AC voltage dividers.

"the situation is a little different, but the result is the same." -- Really?

Kind regards;

By simply calculating it only remains 2 V per 1μF capacitor

It could still be a leakage along the 600 pF outside insulation and to expose the X2 to more.

100% is my answer for your THEORY question.

As others have said, unless you are qualified AND experienced DO NOT go near 3300V equipment.

Leakage through the 600pF will expose the 1uF to higher than rated voltage every cycle since there is no current limiting in the branch. There are far better "no contact" methods of detecting live wires.

Consider also the alternative where your 600pF capacitor (or anywhere else in the branch between the 1uF and line) goes open circuit. You would sense zero across your 1uF and tell everybody that a live wire is safe!!!!

Please make sure that no other CR4 reader is in the area when the <...failure...> takes place. The forum values the contributions that readers make to it.

Hi

See my previous detailed thread below on the subject.

Link

It can be done but safety (both electrical and fire) is a concern for a simple DIY application using the low voltage single phase ac mains voltage with limited (but still very dangerous) fault current capabilities.

For your application I would SERIOUSLY recommend not attempting it at all, because while switchgear manufacturers use this technique for stepping down voltage for live line detection there are many dangers unless you really know what you are doing and how to safely implement it.

Better to use a proper live line detection device designed for measuring the voltage safely.

Here's a broad overview of how the experts do it. Note that the capacitor bridge is custom built into the HV switchgear insulator bushing well, not separate discrete capacitors you can buy over the shelf. There are good reasons for this.

Link

Ok, so you are using a proper voltage detection insulator, still the circuit is extremely dangerous without further research. I am not an expert on the subject but have designed with capacitor voltage dividers for products and work with HV switchgear and would seriously advise caution.

I would NOT personally trust the circuit as you have indicated above as I have said previously, a capacitor in series on its own is a bad idea. Xc is dependent on frequency, so high speed transients on the AC power supply (which can potentially occur even on HV systems) will see the capacitor as a short circuit and will not be attenuated, hence your capacitor could potentially explode if one or more occur. Violently.

Excellent analysis. You're correct; high speed transients will go right through the cap and with 15000 VAC, that's not what one would like to see ... or feel ... or hear!

To the OP - heed the advice or something really bad could happen.

Hi Jack of all trades,

Please see # 20 and # 31.

Thanks and regards;

Hi Stef,

Thank you and Jack of all trades for providing a lot of information. As jack of all trade indicated, the capacitor is a special one made for the detection of live line, built inside an epoxy housing as shown below. The capacitance between the busbar (Line) and the output t is 150 pF in this case.

Regards,

Job.

...in which case matters pertaining to its potential failure should be addressed directly with its manufacturer, as all the blathering traffic on CR4 does is introduce delay into the proceedings.

Telephone?

A friend of mine had disconnected 1000hp,4160 volt motor from the 7200volt switch gear and transformer,and the 4160 switch gear and transformer,and the inside 4160v switch gear.3 sources,in series.Should be off,right? WRONG! He was certain that it was all off,but just for shits and giggles,he put his 600 volt "Wiggy" tester on the line. The tester exploded,spraying him with hot phenolic plastic,flash burning the front of his body, but he survived.Very blessed he was. He fell backwards instead of forward into the live wire.He was back to work in a week,with only minor burns and small shrapnel holes,and flash blinded. Lesson here,always treat every conductor as if it were live,and never take anyone else's word for it.Test it your self,with an approved pre-tested tester.

That is great advice! To get shocked with 120 VAC isn't fun, but most times it's not deadly.

Quite a few years ago, I heard a story of a couple teenagers who jumped the fence of a power distribution center and they hopped up on a transformer. A huge explosion (my friend heard it from about a mile away) and one dead kid and the other severely burned. I'm not sure what they were thinking - there's a tall fence around electrical power equipment and high tension lines???

This isn't 240 or 480 - it's serious stuff.

Hi Stef,

Further to my reply ( # 20 ), I have inserted a GDT and an MOV (Both in series) in parallel with the 0.33 uF capacitor to prevent it from exposing to a voltage greater than 235 (Please see below). I believe that, please correct me if I am wrong, the GDT will spark over and assume a quasi-short circuit condition after which the MOV clamps the voltage down to 145 V, which is far below the voltage rating of the capacitor.

It should be remembered that this is not meant to be a portable tester but installed permanently within a cubicle. In normal condition, the peak voltage across will only be 12.25 KV. If the system is switch ON when the instantaneous voltage is 12.25 KV and assuming that the capacitors are in their fully discharge condition, the impulse time can only be about 0.0000015 micro seconds (Time to charge 63.2% of the capacitors full capacity).

It should also be remembered that there will be surge arrestors protecting the equipment in the distribution sub-station.

Hi Stef,

Please comment on # 31

Regards,

When I worked in high voltage,we were required to test the ground wire on the pole for high voltage, before climbing,in case of a broken live ground wire on pole.We used a simple 1 meter long wand that had a neon light as an indicator.It had internal resistors to limit current and voltage.Very simple and effective device. They were rated to be safe at the line voltage of 13.2 KV. The length of the tester was necessary because of the insulation required between the resistor(s) and the neon bulb to prevent flash over and to keep the operator a safe distance from the item under test. I do not know if they are still available but something similar should be available. Why re invent the wheel when there are approved devices that exist. Do a google search,you will surely find a suitable device.

A good question and a lot of gloomsters an pessimists in the panel...

Here is the answer: you should not use a X2 capacitor because these capacitors have a self healing inner structure and so they loose capacity whenever an arc occurs. This is derived by metallizing a plastic foil, the layer is blowing away around the failure location. Additionally they have higher ESR values than true foil capacitors.

Use a FK type, not a MK type. Or use an additional overvoltage trap (low voltage MOV) in parallel. The safety risk to get an electrical shock is

--> forgotten earthing

and

--> only to relay on the output of the divider for detecting that is no dangerous voltage in the life line.

I went to a sight where a bird(Cockatoo) had landed across a 13,200 volt switch bank. There were 10 switches in parallel and the insulators were spaced and sized to spec.Surprisingly,the bird was in one piece,but it had absolutely no feathers,and had a black smokey looking skin,like it had been held over a smoker. The insulators that he landed on had exploded and had to be specially made at the factory.Downtime cost $1 million per day,and that was just for one department,the bad switch was bypassed and the other 9 continued to function. I have seen shadows of rats,snakes,and other animals inside of transformer cabinets.High voltage is not to be gambled with unless you have a death wish. What you don't know CAN hurt you.If you cannot afford to lose why gamble with even a small chance of failure?Why do you trust advice from unknown sources when there are factory made and certified devices for the purpose. Roll your own at your own risk.What is your location so I can look for a flash in the sky?

100% correct. Be safe!!! GA