Homemade Electrical Equipment

[ericmark]

A core issue that many PIC programmers fail to consider is fail-safes.

A well written program is only as stable as the silicone it's programmed on. As such, it is imperative to include fuses, MCBs, external watchdog controllers... etc

My favorite fail safe system is a high current relay held open by a watcher circuit. If the watcher circuit detects an error OR fails itself, the relay closes, shorting the supply. That then blows a fast blow fuse and removes power to the circuit in one go.

Fubar.

Yes I have used crowbar protection but the point is all fuses and protection is only good if the fuse or protection is itself from a supplier who has said it is suitable for that use. I had an accident which was made worse because wrong colour relays were used. We all knew relays for personnel safety were always yellow or red black relays may be used to protect the machine but not personnel. So since the relay was black in colour it was removed to prevent damage without any alarm bells being rung as to the danger to personnel.

It is so easy to look at a relay and say that black one is half the price of red one I'll use black.

I have also made mistakes because I did not foresee what could happen. I had a safety switch remove the supply to an air solenoid when a door was opened so the ram would stop. What I had failed to realise was the ram may stick and I needed to dump air in cylinder as well.

I have found many safety systems which were dangerous in one case hitting emergency stop could cause one ton of concrete to be dumped on the operator. "Po body is nerfect". So the idea is always to get at least two people involved so something missed by one likely will be seen by another.

Even the big firms get it wrong. My father-in-laws automated garage door which needed to be operated to gain assess to consumer unit had no manual over ride! Ups.

Writing the manual is also a problem. Read the instructions for starting a Model T ford and they forgot to tell you to take it back out of gear and if followed by letter it would run over you.

The problem is the more we protect the more likely to go wrong because people don't watch out for their own safety they assume it will be fail safe. So when coming to a personnel gate on a level crossing they will assume you will not be able to open it if a train is coming. However that's not the case years ago we would have been far more careful.

 

[ban-all-sheds]

The problem is the more we protect the more likely to go wrong because people don't watch out for their own safety they assume it will be fail safe.

I read once (could well have been an urban myth) that the authorities somewhere where there was one of those winding roads up the side of a mountain decided to install Armco to replace, well, nothing, really.

Accidents increased because drivers no longer took as much care as they did when a mistake would have sent them plunging to their death.

However that's not the case years ago we would have been far more careful.

I was being serious here:

Should have had the balustrade and handrail only on one side - nothing like learning about paying attention to ones surroundings and taking care at an early age....

 

[JohnW2]

I read once (could well have been an urban myth) that the authorities somewhere where there was one of those winding roads up the side of a mountain decided to install Armco to replace, well, nothing, really. ... Accidents increased because drivers no longer took as much care as they did when a mistake would have sent them plunging to their death.

Whether or not that one is an urban myth, I don't know, but it's certainly a general problem that adding 'safety measures' can often lead to an increase in complacency, hence potentially more incidents/ accidents/ injuries/ deaths/ whatever. I suppose that RCDs are, at least for some people, an example of that, but there are examples in all walks of life.

Indeed, even moving away from psychology/complacency to biology, there are examples of where well-intentioned 'safety measures' can increase risk - e.g. obsessively 'protecting' babies and small children from 'dirt and germs' leading to less resistance developing and hence more susceptibility to infection. Similarly with 'near (but not quite) eradication' of previously endemic diseases.

Kind Regards, John

 

[i_am_fubar]

An example that is constantly banded about is that if you remove seat belts, airbags and other safety features from a car and put a 6 inch steel spike in the steering wheel, people would drive so carefully, there would be almost no accidents.

Somewhat destracting from the homemade electrics side of things, but the general consensus is that...
1) Designed sensibly and using COTS equipment to interface into 240v circuits is safe.
2) Insurance companies will use it as an excuse to scr3w you.
3) Keep LV and ELV separate by at least 2kV isolation.


Fubar.

 

[JohnW2]

An example that is constantly banded about is that if you remove seat belts, airbags and other safety features from a car and put a 6 inch steel spike in the steering wheel, people would drive so carefully, there would be almost no accidents.

Although that ('almost no accidents') is obviously wishful thinking, a lot of people would certainly drive more carefully under those circumstances - which I suppose is another way of saying that they started driving less carefully due to the complacency resulting from seatbelts and airbags.

Somewhat destracting from the homemade electrics side of things, but the general consensus is that...
1) Designed sensibly and using COTS equipment to interface into 240v circuits is safe.
2) Insurance companies will use it as an excuse to scr3w you.
3) Keep LV and ELV separate by at least 2kV isolation.

That all sounds pretty reasonable/true to me.

Kind Regards, John

 

[ban-all-sheds]

2) Why do they have to know?

 

[JohnW2]

2) Why do they have to know?

That what I was implicitly referring to back on page 1 when I responded to ricicle's suggestion that one should volunteer information about 'home-made electrical equipment' to one's insurer's - as I said, to do so would undoubtedly open up a can of worms, and quite possibly result in a hike of premiums, by an insurer who didn't understand, or couldn't be bothered to try to understand, the actually issues (or non-issues).

Of course, a general problem with insurers is that if one does not voluntarily 'disclose' such facts in real time, if/when it comes to a claim, insurers can nose around looking for 'failures to disclose' which they can try to use as an excuse for trying to wriggle out of paying up (which they will often try, even if the 'non-disclosed' things are totally irrelevant to the cause of the claim).

FWIW, my house is full of 'home-made electrical equipment' about which my insurers know nothing - and I can't say that I'm losing any sleep as a result!

Kind Regards, John