The Forget Me Not challenge throws up opportunities for interfacing with electrical devices.
There are many ways this can be accomplished, but sadly I do see some dangerous practices, that are posted by the ignorant.
Dangers
Electrical safety isn't something to be dismissed, since it can kill.
Generally the victim isn't someone working with the Mains voltage, but an unintentional contact (ie by accident)
Phone kill: Faulty charger electrocutes woman in Australia — RT News
While the above tragic event is linked to poor manufacturing, poor implementation is just as bad and can have the same effect.
This post has a very good video showing the construction of a cheap switch-mode power supply.
http://www.austech.info/electronics/92054-cheap-usb-devise-kills-woman.html#post611192
The important bit to note is that the insulation between the mains voltage and the low voltage side was obtained by a single layer of insulation.
Safety
The basic premise of electrical safety is to prevent unintentional contact with voltages exceeding 50V ac or 120V DC.
In order to achieve that, cables that are exposed to contact must have double insulation.
Photo source Wikipedia Thermoplastic-sheathed cable - Wikipedia, the free encyclopedia
The picture above shows 2 core + earth TPS where the wires contain the first layer of insulation and the TPS Sheath the second.
This cable is generally used for fixed wiring (ie inside the wall cavities and in the roof space, etc), and may have a single strand if the size is smaller.
Cables designed to be moved or flexed and may be subjected to greater abrasion have a different construction featuring a higher number of smaller conductors in each 'core' and tougher sheathing that resists abrasion.
Photo source TLC Electrical Supplies
As you can see there is two layers of insulation BUT if the outer becomes damaged, then it is essential that it is repaired or replaced.
How is this relevant
For this challenge some may be wanting to measure the current used by an appliance, and some practices are NOT SAFE.
Generally the outer insulation is removed, and a measuring device is fitted around the cable, which might be rated (or not) for mains voltages.
This was done by a professional for the purposes of testing AND SHOULD NOT BE USED as a normal practice.
Photo source http://goughlui.com/?p=5184
Placing this arrangement inside an enclosure is marginally better, but the cable leaving the Current Transformer is unlikely to be mains rated and therefore should be 50mm away from cables carrying mains voltages.
Something this person clearly didn't understand when wiring this Death Trap.
Photo credit ...someone with a death wish
Switchboards
There is also a requirement that cabling within a switchboard is suitably rated and because of the 50mm distance, these are not safe.
The photo source is american and the laws may allow this in a domestic situation, but it isn't legal here or Australia.
While the CT may be rated, the cables aren't double insulated BUT could have been sleeved to make this 100% safe.
These clamps may be rated (and of good quality), the cables aren't and the 50mm spacing is compromised, .... especially when you read that he shoves them in to put the cover on.
The exposure on the outside is plain wrong and because of this I'm not crediting the source.
This project is on the OpenEnergyMonitor website and was copied by one of our local members, until we pointed out that the CT cable needed sleeving as it exited the switchboard.
This is a good example where most of the bits are right, but a little more makes it 100% safe.
Photo source http://openenergymonitor.org/
A cable clamp (rather than a knot) on the mains cable on the bottom left wouldn't hurt either, but top marks for putting it inside a plastic case to prevent contact by others.
Really DUMB
This person deserves entry into the Darwin awards.
Not only has he put together something that is dangerous, his comments about why it doesn't work so well shows he has little understanding of what he's doing.
He did have this warning, BUT he should have wrapped the metal bit FIRST then added wire that was mains rated.
Show me a SAFE way
The only safe way to measure mains currents is by observing the double insulation requirements.
At work we had a need to easily measure the currents in our racks, and one of our electricians put together this.
This consists of a normal socket outlet (we use the screw retainer) with two cord clamp modules.
A piece of 10mm mains rated sleeving is clamped and forms a U shape that allows any size clamp meter to be applied.
The neutral wire to the outlet is fed through the loop, before terminating on the socket.
This provides a safe means to measure the current (since the phase and neutral currents SHOULD be the same) and therefore any current meter or CT could be used.
The use of the neutral wire also means if something went horribly wrong, the voltage potential above earth is minimal/zero.
In the next safety blog, I'll show a safe means of controlling an outlet, that is also wireless (RF).
Mark
edit ... the first two links seemed to be joined.
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