safe to work?

[johnmf]

wondered if anybody could answer this.

when changing a socket in your house or wiring in a light you will obviously turn off the mcb cutting live power, however you are still working with the nuetral which is connected to other circuits via the nuetral bar at CU. how come this is safe to do so and is there circumstances/systems where it is not safe to do so??

 

[Steven Barry]

John,

In my experience some of the worst shocks you can get are from a neutral back feed from other circuits, but i think this mainly happens with loads which have an unbalanced supply, i.e motors.

 

[kai]

It is safe to work with just the MCB turned off on a TNCS/PME or TNS system, but you may find any RCD in the CU will tend to trip, if you cut a circuit cable to work on - even with the circuit MCB turned off.

It happened to me recently, I flicked off the downstairs lighting breaker, and proved the circuit was dead with the 16th edition tester, but when I cut the cable, off went the RCD, as it was sensitive enough to detect the cable being cut, even with the MCB off! (Wylex 30mA RCD).

 

[ricicle]

Borrowed or missing neutrals are the main causes of danger when the circuit can appear dead but the circuit is live from the source through the load and will cause a shock when anybody touches it thus completing the circuit

 

[JohnD]

If you were doing it properly, you would test it live with your tester, turn off the power, test it dead with your tester, then test your tester on another live circuit just in case your tester had developed a fault while you were turning the circuit off.

 

[scousespark]

It is safe to work with just the MCB turned off on a TNCS/PME or TNS system.

No it isn't. A circuit is safe to work on when it is isolated (all live conductors deengergised). In a circuit both Phase and Neutral may be 'live'. Isolation should be proven as described by JohnD and has some measure employed to ensure the power is not switched on again.

In a domestic installation the easiest way to do this is switch off the DP main switch and label it so nobody switches it back on.