Safety Equipment and Precautions for DC Circuit Experimentation and Development

Question

What safety equipment and precautions should I use when learning about, experimenting with, and analyzing DC circuits? I have a power supply capable of 32V@5A and 16V@10A (which supports selectable constant current or constant voltage). I'll be working with multimeters and oscilloscopes for analysis while creating circuits.

I don't want to kill myself, burn off a finger, or burn down my house. Not destroying my equipment would be nice too, but that is secondary to not destroying myself. Primarily, what safety equipment should be used? For example, gloves, eye protection, insulated and/or grounding mats, etc.

Answer 1

Well the most important thing is to actually think about everything you're about to do when connecting a circuit.

Check if the oscilloscope probe's ground pin is shorted to the power supply's ground pin (in most cases it will be). Check if output pins of the power supplies are connected to ground pin. They might be, but good quality supplies will have separate ground pin available on front. Also check if the PSU case if connected to ground (it probably will be) and if it is, take care not to have a positive wire touch it.

Every time you connect the scope to something powered by the power supply, think what's going to happen. I've seen cases where people shorted-out their power supplies using oscilloscope probe ground pin and couldn't realize why that happened.

Next, check how the power supplies are going to react in overcurrent situation. Is it going to shut down or will it drop voltage or something else? In general, familiarize yourself with equipment you'll be using.

Do keep in mind that a multimeter in current measurement mode is basically a short-circuit and in voltage mode is basically open circuit. Take care how you connect it! Do read the manual and try to understand what happens in other modes, if there are any. Take note of maximum voltage the multimeter can take with each of them.

For the end, once again, read the equipment manuals and try to plan out each situation in which you can find yourself in as far as the equipment is concerned and think about what will happen if you make a short somewhere.

ABOUT THE COMMENT:

The problem has to do with "floating" and "ground-referenced" power supplies. When a power supply is said to be floating, it means that you can't make a current loop which goes back to ground. An example of this is a battery-operated device. Current goes from one terminal of the battery to another and if you connect one side of the battery to the ground, no current will pass through the wire, because there isn't a closed loop for current to go through. Take a look at this simulation where the ground is connected to the positive output of the battery and no current goes through it.

Same thing happens when you have a transformer separating the mains side of a power supply from the low voltage side of the power supply. All current going out of the secondary side of the transformer needs to go back into the secondary side of the transformer and if you touch a wire along the path, no current should go through you since there isn't a loop for it to go through. Take a look at this simulation. Here too we have ground on the secondary side of the power supply and co current goes through it.

Now to get back to measurement instruments. A hand-held multimeter is often battery powered and is therefore isolated from the circuit it is measuring. This allows you to for example connect the negative probe of the multimeter to the positive pin of the power supply and positive probe of the multimeter to the negative pin of the power supply and the measurement will work, but you'll get negative voltage.

On the other hand, most oscilloscopes are connected to mains power and the ground pin of the probe will usually be connected to the ground pin of the oscilloscope's power supply. On some bench-top power supplies, the negative side of the secondary is connected (or can be connected) to ground. If you for some reason connect the ground pin of the scope probe to the positive pin of the power supply, you can create a short. It will look something like this.

Answer 2

Typically when working up to 50V voltages there should be no danger of dying.

See here: http://wiki.answers.com/Q/What_kills_people_current_or_voltage. To be on the same side it's better to not have more than 1mA flowing through your body (since at 10mA you risk to die). Better not have wet hands either (see http://answers.yahoo.com/question/index?qid=20080204005218AAw90iW).

If you want to have a more subjective information you can take a simple ohmmeter and mesure the impedance between any two fingers. That can give you a rough estimation of your body impedance. The maximum voltage you can apply is therefore given by U_max = 1mA * R_measured.

I actuallly worked on some circuits powered by 15V and - while at first you may sense a little shock - there is nothing excessive with it (it depends on the person sensitivity to voltage).

Adding a fuse might also be a good safe precaution (limit your current to only what you really need). Depending on the voltage source you use, you can configure the maximum output current.

The risk of fire in your house is primarily due to short circuits: avoid them at all cost (place a fuse between your circuit and the power source and / or between the power plug and the power source). That way if something bad happen you won't lose power in the whole house (not sure if that's true in the US, theoretically all houses should have fuses at their entrance, but adding a very fast fuse between the plug and the power supply may avoid losing all electrical power in your house).

Next: all equipment exposed to high voltage should be grounded. In particular your power supply (you don't want to feel the high voltage when touching a power supply enclosed in a metal case).

As far as I'm concerned, gloves are not necessary. You may want to wait for other answers though. A simple test we made at the university was to simply put two of your fingers between the + and the - of a voltage source and slowly increase the voltage from 0 to 32V. I suspect you may feel some trembling at 32V. At 16V you may not feel anything at all or some shock the first time you touch it (which may scare you, but it's not dangerous).

To sum it up: 16V should be safe, while 32V should require some attention (subjective to your body impedance, as well as if you have wet hands). Not so sure if it'd be practical to work with gloves on. At that point better take off all power from the circuit and change whatever you need in it, then plugging the power source in again.

EDIT Beware: check the maximum allowed voltage at the input of the oscilloscope. I'm quite sure the voltmeter supports up to at least 100V. Maximal voltage varies from oscilloscope to oscilloscope. I think it is, but it's better to check it out before burning it down.

EDIT 2 You didn't specify which components you use. Some of those overheat (resistors have a power rating that should not exceeded), others may produce weird smell (typically tantalum and electrolytic capacitors if branched with the wrong polarity), other may explode (transistors mounted in the wrong direction and/or of the wrong type [see Why did my MOSFET explode? and/or subjected to voltage higher than what they've been rated for).

Answer 3

This has been covered in a few questions, but not all together. In summary, and ignoring most of the alternating-current-related questions and answers:

• How much voltage is "dangerous"? & Safe current limit for human contact?
  • 20V & 51mA
  • 60V & 100mA
  • 48V & 1mA
  • 16.5V & 30mA
• How to properly isolate multiple power supplies
  • Don't overload your equipment.
  • Establish a common ground point for all outputs.
• Is it safe to disassemble an unpowered PC power supply?
  • Wait for charged capacitors to discharge or discharge them properly manually.
• How to attach (safely?) an o-scope to various sources
  • Read equipment manuals carefully.
  • Don't measure AC without isolation.
• At what current and voltage do typical resistors explode?
  • Tantalum caps and tranzorbs may explode above 2X their ratings; resistors burn above their power rating.
  • Current limit your supply to the minimum possible (just above required).
• Is it safe to run and charge a deep cycle battery in an enclosed space like a bedroom?
  • Vent hydrogen-producing (lead-acid, etc.) batteries.
• Use a lithium ion battery from a laptop, for a small project?
  • Use under-voltage cutoff, draw below rated current.
  • Monitor and limit temperature when charging (or use laptop to charge).

Insulation, including gloves and matts, should not be a safety concern, but for electrostatic discharge protection. For safety, poweroff and discharge the circuit before working with it. Fuses, breakers and related components are used for fire protection. A multimeter needs good input protection, though primarily for high-capacity AC faults and high-voltage surges (long spikes), as explored by Dave Jones in this EEVBlog video: [link]. Exhaust fumes and wear goggles while soldering. Don't feed the trolls.