# Guidelines for determining shock hazard of capacitors

I'm looking for guidelines on how to identify capacitors which have the potential to cause pain, injury or death due to electrical shock if not handled correctly.

I recently purchased a "getting started with electronics" kit from Radio Shack. It contains an electrolytic capacitor of 1,000 µF and 25 V. I'm assuming this particular capacitor doesn't have the potential to cause such harm because it was included in an introductory kit. However, at what point does a capacitor have the potential to cause pain, injury or death due to electrical shock?

Ideally, I'd like links to reference material on the subject.

First, it is not the capacitor that can harm you, but the voltage and charge stored in the capacitor. So all capacitors are safe when uncharged, which is what they are when you buy them.

To do harm to your body, the voltage across the capacitor's terminals must be high enough to cause a harmful effect on you. There are no hard rules for at what voltage things become harmful, but a common 'rule of thumb' is that DC up to 48 Volt is considered low voltage. So a capacitor charged to a voltage below 48 V is fairly safe.

That does not mean that a capacitor that is rated for 25V is necessarily safe: it is guaranteed to work to 25V, but it is not guaranteed that it won't work up to let's say 70V. And it also does not mean that a capacitor that is rated for 1000V is harmful: it is only (potentially) so when charged above 48V.

There is another form of harm: a capacitor with a very large capacity, charged to an otherwise safe voltage, can cause a very high current when its terminals are shorted. The sparks and heat can harm you, and the capacitor itself could explode. No need to worry about this effect with you garden variety capacitor up to below let's say 1.000 uF, but shorting a capacitor is something you should avoid nevertheless.

• As a general rule of thumb, before sticking your hands in a circuit with potentially charged capacitors, especially power electronics like audio amplifiers and televisions. It is a good idea to short capacitors across a (largish) bleedoff resistor to discharge them. Sometimes there is a bleedoff path already designed for the larger capacitors but I never bet on it. Old CRT's can hold a charge for hours after being unplugged, for instance. Or say rectifier caps, depending on where the circuit is opened by the power switch Aug 4 '13 at 21:35
• Also, just to clarify a point, your impedance to ground across your skin can be relatively high so DC voltages below 48V can be considered "safe", but this is a rule of thumb that is not always true. Blood and lymph is a pretty damn good conductor and your skin impedance can vary based on your hydration, atmospheric conditions, and contaminants on your skin (for instance, an eeg probe on your head can see 100Kohms+, or it can be 5Kohms if you have electrolyte in between the probe and the skin). Aug 4 '13 at 21:42
• And not to belabor the point, but surface area of contact is also important, grab a wire with 15V on it and nothing will happen (most likely), grab the metal plate on the back of a 7905 that is connected to the hot 15V in and you will feel the tingling Aug 4 '13 at 21:53
• 'Uncharged ... when you buy them' is FALSE. I regularly buy 10,000uF capacitors which are invariably delivered with around 5V across them. They are delivered in safe packaging but unless discharged through a resistor before soldering there is a very real risk associated with their initial use. Feb 21 '16 at 16:58

Electrolytic capacitor of 1mF? Well, maybe it can cause you some inconvenience if it explodes with burning electrolytic liquid in your face, but really, nothing to worry about. I'm sarcastic, but I admire your wisdom - the most of us just did things and learned it the hard way.

Another thing you might want to avoid is touching the pins of a fully charged cap when you're wet. Few sources show that the resistance of your body (let alone finger-to-finger) may get down to a few $k\Omega 's$ in such a conditions. Based on the nominal voltage of 25V, this may lead to a current of tens of mA, which this capacitor may sustain for up to a fractions of a second ($Q=CV, I=\frac{dQ}{dt}$). This won't kill you (unless your heart is very sensitive), but it might be very painful.

In general, 1mF capacitor is a BIG capacitor. In general, all electrolytic capacitors are dangerous bastards if not handled properly. It may be said about all capacitors, but electrolytics are special in that they may actually explode. They also very sensitive to reverse polarity voltages - the + terminal is usually distinctively marked. Add the above two statements, multiply it by the fact that you are new to the field, and you'll get the answer to your question - try smaller capacitors before you plug this monster anywhere.

Whether a capacitor can cause injury is mostly related to its voltage rating. If it is not designed or rated to store high voltages, then it won't have enough voltage potential to create a current in a human touching it. Think of it like a battery: You might have a very large current capacity in a battery but if it is only 3 volts, it is extremely unlikely to ever create an appreciable current in a body that is many megohms.

There are many questions about what voltages/currents are required to cause pain and injury. From these you may notice that "high" voltage (for human contact purposes) is generally defined as somewhere around 48 volts. (You can still get shocked from 12V, but given special circumstances.)

The next factor is the capacitor's charge capacity. If the stored charge is at a sufficient voltage to create a current, then any capacitor can be dangerous. The charge capacity will dictate how long the current is capable of flowing. In other words a small value (say less than a microfarad) would result in a very brief shock, whereas a large value (a few microfarads or more) could result in a higher energy discharge, causing a more severe shock, burns, etc.

Extremely high voltage capacitors (1KV+) can actually charge from static in the air, and as a safety precaution they are usually stored with a conductor shorting the terminals. Be extremely careful with any such capacitor.

The true dangers of high voltage capacitors is MULTIPLE CAPACITORS. I have seen some people building their own railguns by plugging in over 100x 9v batteries to a capacitor bank of of almost 20 or more can sized capacitors that can operate at 450 volts. That is when things get really dangerous. When discharged the energy from the capacitor is so intense it can make the areas where all the wiring and rig components are attached to each other explode.