99 percent of homeowners and property managers do not know about this invisible yet extremely hazardous thing.
At issue are two-circuit three-wire-cables like 14/3, 12/3 ... 6/3 ... that share a neutral. Hot conductors of such cables must be on different legs (different phases) of the service so that the max current on the neutral is less than or equal to that of any of the two hot conductors.
If configured correctly, the hot conductor currents tend to cancel out one another in a balanced load, summing to zero neutral current.
If misconfigured, the neutral current doubles up instead.
Or, in other words, if both hots are coming from two breakers on the same buss, phase or same leg in the panel, the neutral carries the sum of both hot currents. When this happens at full load, the neutral can carry up to twice as much current as it is rated for.
This creates one of the most dangerous conditions because neutrals are not protected from overcurrent.
On top of that, this potentially deadly trouble is neither visible, nor obvious. Especially in old panels overcrowded with wires. And a sharp eye of an experienced electrician is required, as well as time to discover such danger.
Shared neutral overload is very similar to overfusing, when a 30 Amp fuse or breaker is used on a 15 Amp line. Too much current (or twice as many Amps) flowing through would burn insulation, ignite an extremely dry dust around it, and cause a major fire along its entire length.
About 10% of panels in homes and buildings have this extremely dangerous fault right now. While nothing bad has happened and may never happen, there remains a possibility of a fire.
Hire an experienced electrician to check your system ASAP to prevent burning or fire.
DIY tips to testing a modern three-wire cable that has a hot black, a hot red, a white neutral, and a bare ground.
Turn breakers on. The correct voltage tester reading between hot black and hot red should be from 208V to 240V. And from 0 V to 1 V testing ground to white.
And you should get 110V to 120V readings:
black to white
red to white
black to bare ground
red to ground
If hots of the same 15 Amp two-circuit tree-wire-cable are connected to two different screws of a double breaker consisting of two full size 15 Amp breakers, it is right and correct but only if there is a 208V to 240V reading between them after such two pole breaker was turned on.
If red and black conductors of a 20 Amp 12/3 line are connected to two different screws of two different 20 Amp twin breakers or tandem circuit breakers, it is right and correct but only if two side by side devices give 208V to 240V.
If two hots belonging to one cable:
a) both go under the same screw of the same breaker, it is wrong
b) connected to two different terminals of the same twin or tandem circuit breaker, with a zero voltage reading between the red and black, it is wrong and dangerous
c) terminated to any energized breakers and give a zero voltage reading between them, it is bad and dangerous
Similarly, the incorrect use of three phase lines with neutral would create an even greater danger, tripling current in the unprotected neutral - an outright deadly situation
For more mistakes and dangers keep reading here: overfusing could burn you alive
Call electrician in Mississauga to detect and fix any electrical conditions or mistakes