# Why cut both live and neutral wires?

It is a common practice, for high power loads such as boilers, furnaces, etc., to install a circuit breaker that "cuts" both live and neutral wires - at least in Europe.

In my search for EVSE, I saw the following figure, in which AC and DC circuit breakers, again, cut both wires.

I understand that it is possible to have a high voltage potential (up to 40 or 50 V) between neutral and earth - due to bad earthing - but is this reason good enough to cut the neutral? Are there any other reasons?

• hint : look for Delta or triangle wiring. Commented Oct 26, 2018 at 13:46
• Plugs like Schuko are symmetrical, there's no defined neutral and live. Either can be more than 100V relative to earth. Commented Oct 26, 2018 at 13:47
• Schuko has nothing to do with this. Wiring in the switchboard is strictly defined. All "low power" circuit breakers must interrupt the live wire. Only "high power" circuit breakers interrupt both live and neutral. Commented Oct 26, 2018 at 13:50
• May have more to do with grounded power system going to ungrounded vehicle. Note fuses on both lines. Commented Oct 26, 2018 at 14:17
• I don't see the relevance of the circuit in your question - the output is isolated and rectified and that means both wires need to be switched. Commented Oct 26, 2018 at 16:56

It is all too easy to get a shock between "neutral" and earth or ground...

Neutral is not always zero, some people make the mistake of assuming it is.

With the effective length of some conductors, then when other devices are running there can be enough voltage present on the neutral to surprise you or cause you to hurt yourself...

So, that is one reason both line and neutral are isolated...

• And even if the voltage difference is too small to cause a shock, it may still be able to source enough current to start a fire. Commented Oct 26, 2018 at 14:57
• I thought that it has something to do with this, but why we use two-pole switches only in high power loads? I believe that the reason also relies in the current that runs through the wires. Commented Oct 26, 2018 at 15:22
• @thece - the voltage on the neutral line is proportional to the current it's carrying. Commented Oct 27, 2018 at 10:14
• @Jules I agree. For the same wire resistance, higher current leads to higher voltage potential between neutral and earth. But for this to happen, switch and appliance must be ON. Obviously, when working with live wires, this "stray" potential is the least of your worries. On the other hand, let's say we cut only the live wire of a high power consuming furnace. Current has no loop to run, hence neutral wire will have 0 A. Only if the same neutral is used by another appliance, this will be potentially dangerous. So I still don't quite understand why only high power loads have two pole switches. Commented Oct 27, 2018 at 17:15
• Surely the voltage produced on the wire is proportional to I^2*R.. Commented Oct 27, 2018 at 17:23

A typical US residential electrical feed consists of two 110V anti-phase "hot" wires and a neutral return wire. This allows for the use of 110V or 220V appliances, and also allows 110V appliances whose total draw is 400A to be fed using three wires that are individually only rated for 200A [as opposed to needing two 200A hot wires and two 200A return wires, or 400A hot and return wires].

If a device designed to run off 220-240V is installed in such a residence, its two power leads will not be connected to hot and neutral, but will instead to the two anti-phase "hot" wires. Disconnecting only one of the wires (it doesn't matter which one) would leave the device sitting at the potential of the other. Ouch. Consequently, it is important to ensure that any time power is cut to either side of a device, it is cut to both.

For devices which will only be used with a ~230V hot and a neutral, rather than a pair of hot wires, disconnecting only the hot wire would be cheaper than disconnecting both wires. Many devices, however, are designed to be usable in countries with a variety of electrical systems, and equipping all devices with a double disconnect may be simpler and cheaper than using a single disconnect for h+n installations and a double disconnect for h+h installations.

Generally you do not disconnect neutral unless it is not bonded to ground or there is a specific worry around sparks (fueling stations)

In the diagram you supply the first Circuit Breaker is an RCD, which detects an imbalance in the line current and the neutral current. These are installed in places where there may be an alternative return path for the current through something/someone which is undesirable. Generally these come in 15, 30 and 100mA ratings.

On the DC side of things you do not have and circuit breakers that I can spot. There are two contactors (large NO type relays) which are switched electrically. The protection for the DC circuit is provided by fuses on both the positive and negative bus in the charging station and in the vehicle. Of interest is that the DC- is not guaranteed to be connected to neutral or ground and could have wild potentials in reference to the AC neutral or ground.

Hope this helps!

Sometime the neutral is broken in LVA breakers for the purpose simplifying or making ground fault schemes on multi-main configurations. See Eaton 4-pole breakers for magnum switchgear.