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?

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    \$\begingroup\$ Use an isolation transformer. That's the best thing to do. \$\endgroup\$ Sep 6 '19 at 20:21
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    \$\begingroup\$ 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. \$\endgroup\$
    – TimWescott
    Sep 6 '19 at 20:31
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    \$\begingroup\$ 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. \$\endgroup\$
    – Russell McMahon
    Sep 6 '19 at 22:06
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    \$\begingroup\$ @RussellMcMahon I'm doing it mostly as a learning exercise to get comfortable working with mains voltage as my experience lacks in that area. \$\endgroup\$
    – Fiebbo
    Sep 6 '19 at 23:21
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    \$\begingroup\$ 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? \$\endgroup\$
    – Russell McMahon
    Sep 20 '19 at 9:01

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 = 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?


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.

  • \$\begingroup\$ North America GFCI receptacles have trip level of 4 - 6 milliamps and trip time of 0.025 second. \$\endgroup\$ Sep 7 '19 at 3:10
  • \$\begingroup\$ @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. \$\endgroup\$
    – Russell McMahon
    Sep 7 '19 at 3:17
  • \$\begingroup\$ 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. \$\endgroup\$
    – mkeith
    Sep 7 '19 at 4:08
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    \$\begingroup\$ @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. \$\endgroup\$
    – Russell McMahon
    Sep 7 '19 at 7:42
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    \$\begingroup\$ @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. \$\endgroup\$
    – Russell McMahon
    Sep 7 '19 at 7:45

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