1
\$\begingroup\$

Forgive me if this question isn't exactly "on topic" for this site -- it's a bit meta.

I am an academic in the UK, and I occasionally design electrical devices that plug into "the mains" in one guise or another. Some of these are (novel) medical devices. We have an internal blinded peer-review process for this, wherein designs that I produce are reviewed informally, and then formally, for electrical safety prior to manufacturing taking place. I find this invaluable: it isn't my job to know a lot about the wiring regulations and can focus on the science I am interested in (which inevitably isn't at 230 VAC).

It is an internal requirement that anything I use on a patient is designed as if it were for sale in the EU, and we meet appropriate standards accordingly. Note that this is pretty common: there are strict limitations on what work one can do on your house without being a qualified electrician, for example, and it's generally a legal requirement that a 'competent person' sign off on anything involving electricity.

Many of the rules I have been asked to follow surprise me in their depth of thought. For example, above a certain current limit, ventilation hole sizes are further reduced on a device to prevent a dropped-in screwdriver welding contacts together. The order of washers and contacts (star, then spring, then contact, then spring, then nut) is specified for binding protective earthing bolts to metal casework panels. Fuses are specified on the incoming live side only, and not neutral (and, in fact The Electricity Safety, Quality and Continuity Regulations state that, as of 31st January 2013, fused neutrals shall not be retained. There is an HSE requirement for fused neutrals cut-outs to be removed within 28 days of being identified.). I could go on.

Occasionally, we will buy something from elsewhere in the world (e.g. the US, China) sold for an international audience, and this committee will declare it electrically substandard, and mandate remedies. Typically these faults relate either to earthing resistance, conductor-to-earth insulation (i.e. it fails a meggar test) or power supply leakage currents. Often our internal standards seem to go beyond those required internationally -- e.g. IEC 60601 vs BS60601.

However, I've noticed that this level of "paranoia" seems to not be the case elsewhere. In particular, DIY stackexchange has examples of wiring that I would naïvely consider to be almost criminal, often with commentators agreeing. What I call RCDs (common on every socket in the country) are only required to be used in the US in "high risk" areas -- e.g. in the bathroom (where no UK house has a conventional 13 A socket at all). I haven't even mentioned how the Type G plug is wonderfully over-engineered, and the average device has two fuses (typically one lower, slow-blow, one higher current and faster one), not one.

[Edit: Some quick examples of what I'm talking about would be:

My question is this: is the UK's paranoia justified? Do we have anything to show for it? Is the rate at which electrical safety incidents occur higher in the United States compared to the UK? Or does the magic difference between 110 VRMS and 230 VRMS obliterate any potential safety gain that this vast amount of work would achieve?

\$\endgroup\$
  • 2
    \$\begingroup\$ Honestly I wish we had RCDs/GFCIs on every socket here, it would be a good idea... \$\endgroup\$ – Hearth Jul 28 '19 at 14:59
  • 1
    \$\begingroup\$ Can't really answer, because I am not usually working with mains at this level and definitely the UK system has it's quirks. Also in your case the device being medical will require extra protection from hazards. But it might be that in general stricter rules cause less deaths, and less deaths is always good, right? Sometimes being sloppy with mains is hazardous - wrong installation like on a street lamp may become hazardous after some years and someone touching it can die. Due to UK ring mains shortcircuit currents are higher, so the fuse in plug really protects the mains cable. \$\endgroup\$ – Justme Jul 28 '19 at 15:15
  • \$\begingroup\$ "where no UK house has a conventional 13 A socket at all" - think again! VTC because it is not relevant to this site and seems to be flawed and asks about paranoia. \$\endgroup\$ – Andy aka Jul 28 '19 at 16:12
  • 1
    \$\begingroup\$ You have presented a discussion with an invitation for further discussion and opinion. This forum is for helping people understand the theory and practice of electrical engineering. That may involve some specific questions about safety, but not a discussion about comparing codes and standards throughout the world. There is an effort toward global standardization, but this is not a discussion forum any any sense. I am voting to close the question. \$\endgroup\$ – Charles Cowie Jul 28 '19 at 16:13
  • 1
    \$\begingroup\$ That’s a really good question, and worth a meta-study. I wasn’t able to find one easily that isolates electrical workers from end-users. My own anecdotal experience with electrical accidents (US) has been with house fires due to faulty wiring on high-load circuits. On the other hand I’ve not heard of a case of outright electrocution from a socket itself due to its design. I have heard of electrocutions from poor SELV-type products with excessive leakage - the stuff that keeps Big Clive busy. My general thought is 110V wiring demands larger gauge and more attention to splices than 230V. \$\endgroup\$ – hacktastical Jul 28 '19 at 16:28
3
\$\begingroup\$

It seems there are catastrophes and there are incidents of leakage current that may cause issues. I don't have any stats.

A catastrophe is an average power (MVA) transformer that blows up every month around the world due to insufficient Condition Based Monitoring. (CBM) for H2 and other pyrotechnic gases dissolved gas in the oil.

The UK uses a much higher safety current for RCD's based on the lethal current thru a hand and not for medical use. The American standard is almost 90% lower or 0.25mA max 100uA typ for GFCI ( don't quote exact values but something like this) What every proficient EE fails to do is a representative Hipot & ESD test above standards to failure using a <100uA current limited Hipot on DC to 5kV to do non-destructive testing yet find the contaminant or component-based BVD of the insulation system.

Then perform step rise voltages for ESD up to 25kV for no incident, operator intervention or functional failure as your specs require.

As far as the star and spring washers are concerned, this is due to lack of understanding of metal fatigue and gas-tight compression required to prevent oxidation for earth bond wire. I once designed a system for Avaya Telecom from C-MAC EMS 20 yrs ago that had paint masking on the sheet metal. When asked by my former "metal-benders" ( sheet metal fab shop), if we could eliminate that I designed a method that UL and AVAYA accepted within 30 days with my test results and a sample for duplication of the test. I used a star washer for the ring lug and two nuts that guaranteed contact between the bolt-nut threads, so the powder-coated painted steel chassis and star washers did not matter as washer was under high compression and gouged into the nuts while the nut/bolt threads in the welded stud ensured a gas-tight oxide-free joint < 1mOhm where 100mOhm was the spec for a 10A cordset.

I suggest you increase your Test Engineering skills and improve your Qualification or DVT skills and apply these to all product designs whether they are yours or not. This includes a full environmental suite of stress tests and a DVT plan/Report that includes all Testing to and then above your regional medical safety limits. Remember to modify the Hipot Probe with a string of high R-values to prevent destruction on test failure yet trip the tester and not discharge the large uJ/mJ of capacitance into the target from Ionization or Voltage breakdown of the component or interface contaminated surface or air gap.

\$\endgroup\$

Not the answer you're looking for? Browse other questions tagged or ask your own question.