SSR failsafe
-
@mrehorstdmd Here in the UK although out domestic supply is 240v, on building sites all mains powered tools have to be 110V which are usually run via step down transformers. I don't know if the following is true but an electrician once told me that the reason is, if you accidentally get hold of a 110V live wire it will bite but you can let it go - (hopefully before it kills you), whereas with 240V your muscles go into spasm and it is impossible to let go. Not my field of expertise but that seems plausible to me........
Edit. Actually, this might be relevant to this thread. 110V site transformers are usually centre tapped so each phase is actually only 55V. If one is concerned about mains voltage heaters here in the UK or Europe, consider using a 110V heater and run it from a centre tapped site transformer.
-
120V can and has killed many as have lower voltages. Depends on the circumstances, it's the amperage across the heart that matters most at less than 600V. As a long time electrician it really bothers me for someone to make light of the potential of 120V.
-
@alexander-mundy said in SSR failsafe:
As a long time electrician it really bothers me for someone to make light of the potential of 120V.
You may enjoy this video then https://www.youtube.com/watch?v=06w3-l1AzFk , a shower heater where the water get in contact with live heater wire.
As for high voltage heated beds, this Voron video has good info, including on thermal protection. https://www.youtube.com/watch?v=8C4jtj0OuE4&t=225
-
@alexander-mundy I don't think the electrician was making light of the danger of 110v, merely pointing out that if you accidentally grab a 110v live wire, you have a chance of letting it go (if it hasn't already killed you) whereas with 240v, the muscles will spasm and so you can't let it go no matter how hard you try.
-
bigclive has a youtube video where he demonstrates that
(and tries to disprove darwins theory at the same time
)joking aside, just don't work on live circuits unless you're trained for it, regardless of voltage
-
@deckingman said in SSR failsafe:
with 240v, the muscles will spasm and so you can't let it go no matter how hard you try.
I remember learning as a kid that if you want to quickly check the presence of main voltage with your hand, you should do it with that outer side of your hand. We also use to test up to 9V batteries with our tongues.
On my printer, I have this https://www.amazon.com/gp/product/B00SJ7Z7DQ connected in series, just in case.
-
What I was taking exception with was working live chassis and not being overly excited about it on a learning forum. I was referring to myself as the long time electrician, over 30 years and I am now the owner of an electrical contracting company. Prior I was an electronics bench technician, signal technician for the City, then signal supervisor. Also an amateur radio operator and from before a teenager mentored by a retired Navy avionics electronics technician.
I don't mention my past as bragging, but to say that all of my teachers taught respect for line voltage as have I to my apprentices throughout the years and to my employees. The worst I have ever had it was accidental contact with 120V. I was probably 10 years old and working on an old live chassis AM radio. It locked up all my muscles and I could not move while it felt like a million ants crawing over me. I was soaked from sweat and in contact with concrete. Needless to say at that point I was overly excited about it. Fortunately I thought of what I had been told and completely relaxed and fell off of it. Should have been using an isolation transformer as we did as bench technicians.I have drawn this too far off topic and will stop here.
-
Many years ago I too worked with live-chassis TVs. I always checked (using a neon screwdriver) that the chassis was connected to the neutral wire (not live) before working on them. What I didn't check, but I now realise that I should have checked, was that the neutral connection was solid. I believe professional TV engineers used an isolating transformer to power TVs being repaired, then they could clip an earth wire to the chassis.
-
@zapta said in SSR failsafe:
@deckingman said in SSR failsafe:
with 240v, the muscles will spasm and so you can't let it go no matter how hard you try.
As a teen, barefoot on concrete, I grabbed a metal lamp that was fully live. USA 110/120 standard. COULD NOT let go. My entire arm was pulsing at 60Hz. My other hand was free and I had the presence of mind to unplug the lamp. Teenagers must have very resilient hearts, same thing would likely kill me now (61 yr old).
I remember learning as a kid that if you want to quickly check the presence of main voltage with your hand, you should do it with that outer side of your hand. We also use to test up to 9V batteries with our tongues.
Exactly, any bite will make your muscles curl the fingers away from the danger. I use that technique to this day, no matter how 'certain' I am that something is 'cold'. It has saved me more than once.
-
@danal yeah the "back of the hand" test is useful in the high power RF world as well for the same reason.
-
There's a perceived safety issue that can exist in persuits like building and maintaining our own machines from freely available plans.
How many people does it take using a machine for how many hours before a machine is safe?
According to this there are 45.5 million registered drivers in the UK (2015):
https://www.gov.uk/government/publications/how-many-people-hold-driving-licences-in-the-uk
And average mileage is around 7900 (2013):
https://www.bbc.co.uk/news/uk-england-28546589
...and 1713 road fatalities in 2013 (including pedestrians, horse riders, cyclists, passengers, bus passengers, etc:
https://www.gov.uk/government/publications/annual-road-fatalities
All that and improving road safety is still a valid exercise.
