Here are some of the ways in which I've seen people mess this up:
- too small a diameter wire to properly engage the thread
- uneven insulation so only the tops get joined
- not enough twist so only the first couple of threads engage
- stranded wire cut through completely by the threads
- wrong kind of wire
(aluminum! which really needs its particular kind of connectors and is fortunately phased out but you may come across it in older installations).
- too many wires for the size nut (usually 5 is the max, depending on thickness)
- re-using oxidized wire ends because there isn't enough wire length
- wires not twisted at all before applying the nut
- leaving out the wire nut spring (!)
(presumably to make room for more wires...)
- spring upside down
- untwisting the nut and the spring, then untwisting the wires to add another splice
- bending the copper too often (metal fatigue)
- not inspected after twisting it on
- too much insulation stripped
- wire end damaged during insulation stripping
- wires twisted in the wrong direction
And so on... In comparison some of these will apply to Wagos but in general they have fewer ways of doing things wrong and adding another splice is much easier with a Wago. DIY electrical is great, it saves a ton of money. But the kind of stuff you come across can make your hair stand on end. The house I live in came with an electrical installation that was downright dangerous, I live here for 5 years now and I think I've fixed most of it but every now and then a new surprise pops up. Recently: a wall socket stucco'd over but before they stucco'd it over a piece of flex wire (twin strand) was connected through a groove cut into the wall to a brand new in-wall grounded socket of which the ground was left unconnected. Whoever made that (I suspect the previous occupant) is a complete idiot when it comes to electricity.
- too small a diameter wire to properly engage the thread
- uneven insulation so only the tops get joined
- not enough twist so only the first couple of threads engage
- stranded wire cut through completely by the threads
- wrong kind of wire
(aluminum! which really needs its particular kind of connectors and is fortunately phased out but you may come across it in older installations).
- too many wires for the size nut (usually 5 is the max, depending on thickness)
- re-using oxidized wire ends because there isn't enough wire length
- wires not twisted at all before applying the nut
- leaving out the wire nut spring (!)
(presumably to make room for more wires...)
- spring upside down
- untwisting the nut and the spring, then untwisting the wires to add another splice
- bending the copper too often (metal fatigue)
- not inspected after twisting it on
- too much insulation stripped
- wire end damaged during insulation stripping
- wires twisted in the wrong direction
And so on... In comparison some of these will apply to Wagos but in general they have fewer ways of doing things wrong and adding another splice is much easier with a Wago. DIY electrical is great, it saves a ton of money. But the kind of stuff you come across can make your hair stand on end. The house I live in came with an electrical installation that was downright dangerous, I live here for 5 years now and I think I've fixed most of it but every now and then a new surprise pops up. Recently: a wall socket stucco'd over but before they stucco'd it over a piece of flex wire (twin strand) was connected through a groove cut into the wall to a brand new in-wall grounded socket of which the ground was left unconnected. Whoever made that (I suspect the previous occupant) is a complete idiot when it comes to electricity.