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Relay switch - Live or Neutral to connect?

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5 hours ago, enl said:

In general, the HOT current carrying wire goes to the relay COM, and the NEUTRAL (grounded) is left unswitched.

Thank you for the reply and guidance. I am not sure whether I understand what you mean by the NEUTRAL is unswitched. HOT comes into COM, where does the NEUTRAL wire goes to ? Are you saying NEUTRAL and EARTH are not used/connected into the relay ?

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2 hours ago, Softwaremaker said:

Are you saying NEUTRAL and EARTH are not used/connected into the relay ?

They are not connected to the relay. They go directly to the load.

 

To turn the load on or off, you need only interrupt one current carrying wire. You usually want to interrupt the HOT lead, so when the switch is off, no part of the controlled device presents a voltage that is a hazard. (This, of course, does not consider parts that store energy, like capacitors)

 

(If you have further questions, ASK. Better to ask than someone get hurt or property get damaged. But please do tell us where you are located. The systems in the US/Canada, UK, rest of the EU, India, etc, all differ, and often differ within a single country based on region)

 

Background:

 

The underlying assumption here is that the source has one line that is essentially at ground (earth ground), and the other is HOT, meaning it it at 120V or 240V (AC RMS) relative to ground. In the US and Canada, this is ensured by connecting one wire at the service entry to the earth ground (this is called BONDING), such that one current carrying wire is within a fraction of a volt of ground (called the NEUTRAL-- formerly called GroundED in the US), the other is 120V RMS relative to ground. Similar systems are use in much of the world. For example, in most of the EU (and UK, which may soon be a separate entity), one stab is grounded at the entry, and the other is therefore 230V relative to ground. When there isn't a stab for the earth ground connection (safety ground), there is some provision in all modern systems. If I recall, the common one in the EU is  the bracket (clip?) on the perimeter of the plug. The intent is that the EARTH GROUND (safety ground) only carries current when there is a problem, and that the safety device (fuse, circuit breaker, ground fault interrupt) should interrupt the circuit when that happens.

Since the NEUTRAL current carrying wire does not present a safety issue, as it is near or at ground potential, we let it be. The GROUND (safety ground) conductor should NEVER, EVER be interrupted while EITHER THE HOT OR NEUTRAL is connected, which is why modern plugs have a longer stab for this. It connects first, and disconnects last. The line that should be switched is the HOT, which is not at or close to earth ground potential.

 

I could (obviously) go on about why it is done this way, but I will cut it short by saying that it is the easiest way to minimize the number of ways that a fault can occur, and make it practical to protect against faults.

 

*********************

Public safety message: bond your grounds properly. When using a portable generator, ALWAYS use a ground fault interruptor, be sure the ground is connected to a good ground tie (driven rod, metal building frame, etc) and the, when powering a building or gear in a building, tied to the building earth ground bus.

 

 

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