If it is from the mains, there are a few things that are known to make things worse...

1. "spider" wiring - typically done on domestic lighting circuits up to the 1980s/1990s - where sheathed single core (or single +c.p.c.) cabling was used - with L looping at the switches and N looping at the lights. In theory it can be arranged so things balance in close proximity (as if it has been run in conduit) but in practice each cable  tended to take the shortest most convenient work and balancing was negligible.

2. Ring circuits - in theory L & N currents should balance in each leg, but in practice small differences in resistance between legs, can create loops over quite a large area. Broken conductors in a ring can make the situations a lot worse, and can go unnoticed.

3. Unscreened cables - using shielded cables - e.g. FP or BS 8436 cables that have an earthed aluminium tube surrounding the live conductors - can help a bit.

4. Diverted PEN currents flowing through extraneous-condutive-parts - most likely in muti-occupancy steel framed buildings with multiple PME supplies, but can happen elsewhere too.

   - Andy.

If it is from the mains, there are a few things that are known to make things worse...

1. "spider" wiring - typically done on domestic lighting circuits up to the 1980s/1990s - where sheathed single core (or single +c.p.c.) cabling was used - with L looping at the switches and N looping at the lights. In theory it can be arranged so things balance in close proximity (as if it has been run in conduit) but in practice each cable  tended to take the shortest most convenient work and balancing was negligible.

2. Ring circuits - in theory L & N currents should balance in each leg, but in practice small differences in resistance between legs, can create loops over quite a large area. Broken conductors in a ring can make the situations a lot worse, and can go unnoticed.

3. Unscreened cables - using shielded cables - e.g. FP or BS 8436 cables that have an earthed aluminium tube surrounding the live conductors - can help a bit.

4. Diverted PEN currents flowing through extraneous-condutive-parts - most likely in muti-occupancy steel framed buildings with multiple PME supplies, but can happen elsewhere too.

   - Andy.