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mrajgolikar
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3P or 4P ACB in Outgoing Feeders for TN-S system

08/21/2019 2:49 PM

In a L.V. Switchboard for TN-S system, apart from the Incomers & Bus-Couplers need to be 4P ACB, is it necessary to have 4P ACBs for Outgoing feeders as well? If so why?
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SolarEagle
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#1

Re: 3P or 4P ACB in Outgoing feeders for TN-S system

08/21/2019 3:22 PM

Depends on the load characteristics and the grounding scheme...

https://cr4.globalspec.com/thread/26706/3-pole-or-4-pole-or-TPN-ACB-Air-circuit-breaker

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raghun
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#2

Re: 3P or 4P ACB in Outgoing feeders for TN-S system

08/22/2019 1:32 AM

If the load is 3-phase balanced load such as a motor, there is no reason why you need to take neutral to the load. In such a case, the ACB can be three pole.

Idea is not to allow neutrals to be mixed up at any location.

Secondly, when there are single phase loads directly connected to the switchboard, it is possible that the neutral can assume some voltage due to unbalanced loading condition (among three phases) and hence needs to be isolated while shutting down the load downstream, for reasons of safety.

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mrajgolikar
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#4
In reply to #2

Re: 3P or 4P ACB in Outgoing feeders for TN-S system

08/23/2019 3:14 AM

Thanks. There are a few lighting loads distributed equally & normally balanced.

The neutrals are no getting mixed up as Incomer & Bus-Coupler ACBs are 4P.

However in a given Bus section, since Outgoing feeders are all 3P & Neutral continuous, when any Outgoing feeder(3P) is switched OFF for maintenance, will any unbalance or earth fault in some other O/G Feeder in the same bus affect someone working on the neutral although the supply is OFF.(in the feeder under shutdown)
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PWSlack
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#5
In reply to #4

Re: 3P or 4P ACB in Outgoing feeders for TN-S system

08/23/2019 3:28 AM

An <...earth fault...> will only persist until the circuit protective device(s) disconnect the fault.

<...unbalance...> causes current to flow in the neutral conductor and it can achieve voltages, which are a function of the current flowing and the earth loop impedance. Many installations have a neutral link that is opened to enable persons to work. Downstream of the link it is possible, and desirable if not mandatory, to apply an earth connection prior to working.

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TonyS
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#3

Re: 3P or 4P ACB in Outgoing feeders for TN-S system

08/22/2019 5:42 AM

On TN-S systems the neutral is kept permanently connected. Neutral test links are fitted on larger breakers and switches.

UK private networks have to be TN-S.

The UK LV distribution system has degenerated to TN-C-S, a retrograde step in my opinion. The colonials had this system for years, the use of multiple low kVA “pole pigs” instead of large high kVA district transformers.

Something I’ll never agree with is linking MV and LV neutrals.

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