When to use a 2 pole circuit breaker over 1 pole

Question

We are currently designing a product that connects to 230VAC mains power (single phase) (South Africa). It has a simple 12v switch mode power supply that runs the electronics. We want to put a Circuit Breaker between the mains and the PSU.

What I want to ask is, under what circumstances would one install a single pole circuit breaker vs a 1P+N circuit breaker?

The purpose of the breaker, is just to ensure that should any of our equipment short, break, get hit by lighting etc that the breaker will trip, and not affect other the main power in a negative way. I am essentially using it as a re-settable fuse.

We are considering the following two circuit breakers:

1. Single Pole
2. 1P+N

The 1P+N is roughly 4 times the price of the single pole.

This cost saving could drastically effect the product price, however I do not want to compromise on quality & safety of the product.

Maybe someone could also clarify the difference between a 2P (2 pole) and a (1P+N).

I generally like to install the 2 pole in my own equipment, that way when i'm working on the unit, I know that it is totally isolated from main power. But I feel this is overkill for a product in the field.

Answer 1

I see a few important questions to ask yourself.

• What is the breaker's job?
• How is the product connected to the mains?
• What standards does the product need to comply with?

If the breaker's only job is to protect against overloading a single pole device is fine, but if (part of) the breaker's job is to protect against faults to earth then you should consider the possibility that your supposed "neutral" is not actually a neutral.

For a single-phase plug in device the possibility of your supposed "neutral" not actually being a neutral is considerable. Even if your country's sockets are polarised people may take the device abroad, may connect it via a chain of adapters that don't maintain polarity or may connect it to a supply that doesn't even have a neutral.

So for a plug in device if your breaker needs to protect against faults to earth then it should be a double pole breaker.

A "single pole plus neutral" breaker that isolates both poles but only detects overcurrent on the live is useful when the breaker needs to protect against overloading and provide a means of isolation, but either does not need to protect against faults to earth or is used in a situation where applicable standards consider the possibility that neutral won't actually be a neutral to be negligable.