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Specifically, I am planning to design and build an energy meter to measure power drawn from mains - 120V 60Hz. What are the risks associated with developing and testing this device and how to minimize/eliminate them?

Fiebbo
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    Use an isolation transformer. That's the best thing to do. – Marko Buršič Sep 06 '19 at 20:21
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    If you can't find any references on the 'net, get a copy of the ARRL Handbook -- they've got a chapter on shop safety and good practices. Keep your off hand in a back pocket any time you're probing, and always think about whether the electricity has a path to ground that goes through your heart. – TimWescott Sep 06 '19 at 20:31
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    I' m not suggesting you shouldn't do so BUT WHY are you developing such a meter. Ones that cost less than you can build are readily available. **Unless** you wish to learn from reinventing the wheel or have a VERY special need there are better approaches. – Russell McMahon Sep 06 '19 at 22:06
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    @RussellMcMahon I'm doing it mostly as a learning exercise to get comfortable working with mains voltage as my experience lacks in that area. – Fiebbo Sep 06 '19 at 23:21
  • @MarkoBuršič Isolation transformer seems like a good idea for powering the electronics but I still need to measure the current and voltage drawn by a load. The question is more focused on risks and safety concerns rather than implementation though. – Fiebbo Sep 06 '19 at 23:25
  • @TimWescott that's a great suggestion, would you happen to know of any free references? all I could find online, so far was this [post on high DC voltage](https://electronics.stackexchange.com/questions/34934/safety-precautions-working-with-high-voltages-nixie-clock/34937#34937) – Fiebbo Sep 06 '19 at 23:32
  • An isolating transformer has pros and cons. With it you cannot sustain a mains to true-ground shock BUT you can still sustain a mains to system ground shock AND the latter is now not protected by a mains side GFCI. A GFCI is very good for mains to true ground but useless for Mains to return ground shocks. – Russell McMahon Sep 07 '19 at 01:24
  • You may find ARRL handbook for radio amateurs in a library. You may also find a used prior edition for sale at a low price. A GFCI 120 V household outlet will provide good protection for a shock between hot and ground, but an isolation transformer would be preferable. It is important to follow the rules. Avoid working on a live circuit. Always check to make sure a circuit that you think is not live is truly not live. Avoid having both hands near the circuit. Attach the common side of the meter securely too common. Then use the other probe with one hand. Be aware of nearby metal objects. Etc. –  Sep 07 '19 at 01:35
  • The main rule when developing and testing is to be careful. Don't touch the mains voltage while the device is energized. Clearly segregate the mains voltage area from the low voltage area by design. Cover the mains voltage area whenever you are not probing it, and probe it carefully when you must. At the design stage, think about what needs probing and how you will probe it. If possible, attach and disconnect probes when device is de-energized. For sure the outlet should have GFCI protection (In this day and age, every outlet in your home and shop should have GFCI protection). – user57037 Sep 07 '19 at 03:57
  • What they say. Also, be aware of the "Oh. I'll just change this ..." impulse - the sudden decision to touch something without a well formed thought re the hazards. Developing a very conscious "this circuit could kill me" attitude and taking steps to minimise open hazards is a good start. Maybe 50 years ago I welded a screwdriver to a mains bearing tag when I braced it on an earthed chassis- it welded to the tag and the arc burned the chassis contact clear ! :-). Some years before that I reached out to move something which had 1000 volts+ on it. Lessons learned. Fortunately. – Russell McMahon Sep 07 '19 at 07:50
  • @MarkoBuršič See my answer - note that an isolating transformer does some good things BUT you can still get a live tom local ground shock and, if you do, a GFCI on the input mains circuit then does no good at all. – Russell McMahon Sep 08 '19 at 09:15
  • This question was put on hold for being too broad, so I have edited it. Please let me know how I can improve the question. I was never exposed (beyond a theoretical level) to mains voltage through my EE undergrad. I also couldn't find a definitive safety guideline online. I thinks its a very important question and the information should be readily available for newbies/hobbyist who want to play around with mains voltage. – Fiebbo Sep 08 '19 at 11:41
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    I have added a link to my answer from an SE 2011 post [How is using a transformer for isolation safer than directly connecting to the power grid?](https://electronics.stackexchange.com/a/17500/3288) – Russell McMahon Sep 20 '19 at 09:01

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This is such an important and useful topic that I'll post a quick subset answer to related issues now and try to get back for a much more complete answer later if it looks useful.

To start - a note on GFCIs and Isolating Transformers:

Isolating transformers and GFCIs both have potential (pun noted) value but also introduce new considerations.

GFCI

GFCI = Ground Fault Current Interupter.
Also known as an ELCB and GFI and ... .

  • Current that is carried on both leads must be equal and opposite within some set limit.

  • If current supplied via the GFCI on phase does not return through the GFCI on neutral then the device disconnects power. Current trip limits are usually in the few 10's of mA range with a trip period shorter than the period required to cause ventricular fibrillation.

  • A "shock" via a GFCI as it trips will still be felt as a substantial electrical shock (ask me how I (purposefully) know), but it is brief.

A GFCI is very good for mains to true ground shock BUT if the "ground" contact is to the neutral side of the GFCI circuit then the user becomes "just part of the legitimate load" and there is zero protection.
ie When working on mains powered equipment a mains to neutral fault rather than mains to ground will not be protected against.

Isolating transformer

An isolating transformer is a 1:1 transformer with electrically isolated primary (mains input) winding and secondary (mains voltage) winding.

An isolating transformer has pros and cons.

  • With it you cannot sustain a mains to true-ground shock as there is no electrical path between any point on the secondary winding to mains-ground. (There may be minor unimportant coupling via inter-winding capacitance.) BUT

  • You can still sustain an isolated mains level voltage shock from isolated-phase to isolated system ground AND

  • Such a shock now wholly not protected by a mains side GFCI

See also this SE Q&A from 2011
How is using a transformer for isolation safer than directly connecting to the power grid?

Shock:

Electrical shock than can obviously be via the heart "should be avoided" but any electrical shock has the potential to cause electrocution. eg a foot to foot shock MAY still cause electrocution.

Many substantial electric shocks do not kill (I've had a few over a long lifetime) but any one might. Arrange work practices and mental attitude to not have them.

Be wary of and look for situations that may lead to accidental shock - exposed conductors, casual attitudes, faulty equipment. (I had not had a major shock for maybe several decades - then a few years ago got caught by a faulty switchboard fuse holder that had missing insulation covering a phase-contacting screw - and conductive material had accumulated in the hole. I received a substantial hand to hand shock across the chest with substantial muscle soreness at the arm chest boundary on both arms due to muscular contraction.

DC shocks are potentially more dangerous than AC shocks as they can prevent muscular "let go" - under DC muscles clamp and do not release and it can be physically impossible to release one's grip.
With high conductivity skin contact a voltage of 12 Volts can be enough to cause almost unreleasable muscular contraction. (I have a friend who experienced this!. This is unusual - but all the more worth noting so as not to be 'caught out' in exceptional circumstances - Most people don't get to stand in seawater holding a metal handled flounder spear and a faulty LED 12V lantern mounted on a metal pole, as my friend did :-) ).

ELV DC systems at "safe" highly lethal voltages are increasingly common.
People have died from 36V helicopter battery contact.
Solar 48V systems are potential killers - but often an across the hand 48V shock is usually a mere nuisance 'tickle'. (During under-graduate experience training I worked on a 48V telecom wiring frame. In humid weather brushing the terminals of live circuits often resulted in annoying across hand mini shocks, with no other affects. Technicians in 48V powered exchanges worked on live equipment all the time with no warnings re shock hazard. 48V electrocution is rare. Don't be the exception.

In very worst case conditions 12 VDC shocks can kill. This can occur if voltage is applied across the chest with high conductive contact - unusual but not impossible (and has happened :-( - this occurred during an experiment when a prisoner agreed to participate in a medical experiment. Death was not intended, but happened. Microamp level currents under the skin & into the heart area can kill. [I'll add a reference 'later']. ).

With an AC shock it is often easier to release grip due to the cyclical zero crossings of current allowing muscles to repeatedly briefly cease contraction, but sometimes it is still "almost impossible" to release grip.

Russell McMahon
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  • North America GFCI receptacles have trip level of 4 - 6 milliamps and trip time of 0.025 second. –  Sep 07 '19 at 03:10
  • @CharlesCowie Thanks. There are a few standards. I tried to err on the general side. I have been surprised how high trip current some allow. 6 mA sounds good. 25 mS is intended to be inside the ventricular fibrillation trigger period. I'd not be sure that was always safe (having had some xperience with heart funnies) but they do seem to do a good job. I long long long ago tested a GFCI across the back of my hand to see what it felt like. Briefish - but still a not nice shock. – Russell McMahon Sep 07 '19 at 03:17
  • I don't doubt what Russel is saying about low voltages. HOWEVER, 48V is often considered touch safe in dry conditions. 12V is often considered touch safe in damp conditions. I have worked very extensively with 12S battery packs (~48V) and have not ever had a shock from skin contact (in dry conditions). Again, this is not a matter of disputing Russel, but trying to put the risk in perspective. For sure it is good to understand what the "worst case" conditions are that might increase the hazard of lower voltages. – user57037 Sep 07 '19 at 04:08
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    @mkeith Thanks. I agree with your comments. I've added both extra warnings AND extra disclaimers re rarity. I've worked on voltages up to somewhat over 1 kV (low voltage in real-world terms :-) ). I don't recall getting shocks at under 50V EXCEPT the wiring frame events. I have had various AC mains shocks and DC shocks at up to 1200 VDC! (one only of those!) over 55+ years. Many of those COULD have killed me. I've had almost zero shocks in decades but my perhaps worst mains one was under 3 years ago due to a faulty fuse holder. Life, and death, happens. – Russell McMahon Sep 07 '19 at 07:42
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    @mkeith On occasion on a hot day I've carried a 12V car or light truck lead acid battery. Sweaty and no shirt sometimes. It has occurred to me that with a heavy battery one could easily enough accidentally repeat the 12V across the chest with good conductivity experiment! I've avoided doing so. – Russell McMahon Sep 07 '19 at 07:45
  • @RussellMcMahon I feel more confident after reading your answer. Are you still planning on making it more complete? I've also slightly edited the question to remove the hold on it. – Fiebbo Sep 08 '19 at 19:01
  • @Fiebbo I'll try to get back to add to it. || On the fly: If using conductive antistatic mats be sure not to place a board with mains (or other high voltage) on the mat directly :-) - ask me how I kno :-). Some mats are very high resistance even though ESD dissipative and are close to being an insulator. Some, such as Butyl Rubber - are loaded with carbon to establish ESD safety- AND have low per-square resistance. Smoke can happen (magic and other). || resistor voltage ratings independent of dissipation, X & Y cap voltage surge peak ... ratings, ... – Russell McMahon Sep 09 '19 at 07:09
  • @Fiebbo . [Life gets busy. Remind me in a few days if I've not been back.] – Russell McMahon Sep 09 '19 at 07:10