Neutral disconnector: what is it for? - Page 1
Forum section: electronics / beginners

author: crgarcia
author_rank: Regular Contributor
timestamp: November 12, 2023, 08:32:57 pm
content: I want to build a board that will help me test my PCBs.To do that, I started looking into circuit breakers and is when I found that there are breakers where the phases can be disconnected without disconnecting the neutral.There are alsoneutral disconnectorsthat I guess are used to break all phases and neutral at the same time.For example:https://new.abb.com/products/es/2CCS400900R0011/nt45163?CH#availabilityWhen do you need to open the phases and leave the neutral connected?When do you need to use a neutral disconnector?Thanks!

author: Thunderer
author_rank: Regular Contributor
timestamp: November 12, 2023, 09:01:57 pm
content: Depending on the country, interrupting the neutral, even if the phase(s) is(are) interrupted at the same time, may be forbidden. The case here in the land of maple trees.Not a good idea to cut the neutral from a protection point of view. Why ABB proposes this device is not known to me.You got an answer here (https://electronics.stackexchange.com/questions/688210/what-is-the-purpose-of-neutral-disconnect-in-a-circuit-breaker). But still find it unusual to do it.

author: TimFox
author_rank: Super Contributor
timestamp: November 12, 2023, 10:12:59 pm
content: That ABB unit apparently opens the three phases when the blue lever disconnects the neutral.I assume it will not allow the phases to be connected while the neutral is disconnected.

author: madires
author_rank: Super Contributor
timestamp: November 12, 2023, 10:19:31 pm
content: Quote from: crgarcia on November 12, 2023, 08:32:57 pmFrom a technical point of view it's based on the type of wiring, i.e. 3-wire (single phase) with L, N and PE or 2-wire (single phase) with L and N. In the latter case N (neutral) is usually used also as ground/earth and shouldn't be disconnected for safety reasons. WIth a separate wire for PE N can be disconnected. Anyway, check your local electrical code about which variant is allowed.

author: themadhippy
author_rank: Super Contributor
timestamp: November 12, 2023, 10:30:50 pm
content: QuoteFor testing is the main reason,but also when the regs class the neutral as a current carrying conductor and the same regs require an isolator to break all current carrying conductors

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: November 12, 2023, 10:34:55 pm
content: Hi crgarcia,...looks to me like an auxiliary contactand it's not even properly fitted. B-(

author: soldar
author_rank: Super Contributor
timestamp: November 12, 2023, 11:31:26 pm
content: Quote from: crgarcia on November 12, 2023, 08:32:57 pmI think you are mistaken and misinterpreting what that device does. It seems to me the trip lever goes into the trip lever of the device next to it and when the three phase breaker trips it will mechanically trip the neutral. That's the way I see it.

author: DavidAlfa
author_rank: Super Contributor
timestamp: November 12, 2023, 11:57:27 pm
content: If the neutral goes off, then it'll become an unbalanced delta 3-phase circuit, the lucky ones might get little difference, others 400V instead 230.Happened quite some years ago when I was working at a new building as spark man (Doing homes wiring), the grinder worked quite fast that morning, too crazy fast!We found it was around 340V, seeking the fault we found a burned neutral connection.The neutral will carry zero current if the las over the 3 phases is the same, like when powering 3-phase heaters, motors... The neutral does nothing, and they usually don't use it.So this design forces disconnection of all phases when opening the neutral to prevent exactly this.

author: wraper
author_rank: Unknown
timestamp: November 13, 2023, 12:20:01 am
content: From the datasheet for other modelhttps://www.purvisindustries.com/ASSETS/DOCUMENTS/ITEMS/EN/ABB_S2C_NT_Datasheet.pdfI see:QuoteThis let me think about IT earthing system where neutral is not connected to earth. I guess it would make sense when used in that. I have no idea about actual requirements for IT system and it's not easy to google any detailed info about it.