neutral

[richerdick]

i have been asked what are the safety reasons why most three phase isolators dont break the neutral. My response was that the neutral is held to earth potential and therefore is used in testing for isolation also keeping it connected prevents the neutral from floating to a higher voltage . Are there any other reasons that ive missed? .

 

[Skogsgurra]

That, I think, applies more to the PE. N is disconnected in some so-called five conductor systems aka TN-S. Especially in hospitals.

The reason why N is not disconnected in most installation is probably historical/economical: "If it works disconnecting the hot wire, why should we bother to disconnect neutral?"

Pls note that I am writing from the eastern shore of the Pond. You may see it differently from the western shore.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

In North America there are a few reasons. One is cost, why use a 4 pole switch when 3 poles will do? The cost really goes up when you are looking for a large, non-standard 4 pole switch.
The other is code and approvals. If the neutral is to be switched, the switch must open the neutral simultaneously or after with the hot lines are opened. The reverse when closing. This is a code requirement. Most multi-pole switches open all the poles simultaneously but they are not tested or certified as complying with the code requirement for neutrals. If the AHJ calls you on this you may be looking at unbudgeted extra expenses.
A few real word instances;
A friend of mine was purchasing a large transfer switch for a project. He was out of the office and his supervisor decided to help him out. The super chose the lowest price from the stack of quotes and put in the order. My friend had rejected that quote because it did not include an early make/late break neutral as the project specs called for. That was a $2000 option, and that was at 1984 prices.
A small utility asked me to conduct an investigation of an incident. A customer had lost some expensive communication equipment due to over voltage and was blaming the utility.
I found that the customer had had a manual transfer switch installed for use with his standby generator. The installer had used a 3 pole switch and switched the neutral on a 120/240 Volt service. The cheap manual switch was opening the neutral first and the resulting open neutral circuit had caused the over voltages that destroyed his radio installation. Using a $20 switch in place of a $200 switch. This was in the third world where no codes applied.
The utility paid me for my time trouble shooting and the customer dropped his claim and paid me to connect the switch properly.


Bill
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"Why not the best?"
Jimmy Carter

 

[PQ101]

As you know in US we dont isolate the system neutral on 3-pole switches because we are not required so. But in Europe the codes requires total power isolation on all 3-phase, plus the neutral connection, just to make sure there is no chance for incidents or electrical shock for safety reasons i.e. electrical power system shutdown and locked-out, and totally dead.