MLO LV Switchgear

[philly]

Does anyone typically come across 480V LV switchgear that is MLO without a main breaker. I recently did a project where the main breaker was omitted due to space limitations on a unit substation and in its place were CT's and an overcurrent relay which transfer tripped the upstream MV feeder breaker.

I'm assuming that the secondary CT's and relay provide the required secondary transformer and cable protection as required per the NEC. (I don't know if secondary transformer protection is even required due to this being in a supervised location).

The part that is unclear to me is the requirement in 225.31 and 225.32 that require a building to have a disconnecting means if this MLO switchgear is located in a separate building. 225.32 exception 1 states that if the facility has qualified personell then the disconnecting means can be located elsewhere and is not necessarily required to be located at the Switcghear in the building. Does this mean that this MLO switchgear is ok in a separate building in a supervised facility.

Also in terms of Arc Flash protection I believe that transfer tripping to upstream breaker will provide the same level of AF protection as would a LV main breaker.

Would the secondary CT's and relay also meet the requirement for ground fault protection in 230.95?

 

[Cow]

We use MLO gear quite a bit if we can keep it 6 handles or less. Saves on the cost of a GFI main and testing.

 

[publicgood]

(I don't know if secondary transformer protection is even required due to this being in a supervised location).

If we are talking about delta:wye, protection of the secondary conductors is needed. Not the case here, but True: could be omitted; however, I believe it needs to be an industrial location, not a generic supervised location. 240.92(E)

225.32 exception 1 states that if the facility has qualified personell then the disconnecting means can be located elsewhere and is not necessarily required to be located at the Switcghear in the building.

You are missing pieces to the code reference. It is not just qualified personnel.

Exception No. 1:  For installations under single management, where documented safe switching procedures are established and maintained for disconnection, and where the installation is monitored by qualified individuals, the disconnecting means shall be permitted to be located elsewhere on the premises.

Even still, if there is a primary breaker on this unit substation, this would take care of the disconnecting means. If it wasn't a unit sub, up to 6.

Would the secondary CT's and relay also meet the requirement for ground fault protection in 230.95?

If there are indeed CTs for this purpose, yes.

 

[publicgood]

I recently did a project where the main breaker was omitted due to space limitations on a unit substation and in its place were CT's and an overcurrent relay which transfer tripped the upstream MV feeder breaker.

What you describe likely meets 225.38(A) as a power operable means. Not commonly done, that I have seen, but meets code.

 

[philly]

We use MLO gear quite a bit if we can keep it 6 handles or less. Saves on the cost of a GFI main and testing.

In order to use MLO with 6 handles or less the combined ampacity of the breakers need to still provide transformer secondary protection and secondary cable protection correct? In a lot of industrial applications this usually is not the case as the combined ampacity of the feeders can exceed these requirements.

If we are talking about delta:wye, protection of the secondary conductors is needed. Not the case here, but True: could be omitted; however, I believe it needs to be an industrial location, not a generic supervised location. 240.92(E).

I was never aware of the allowance you pointed out in 240.92 for supervised industrial locations to allow the primary device to protect the secondary conductors. Would this essentially be confirming that the primary device is below the cable damage curve and has a rating equal to or less than the secondary conductor ampacity reflected to the primary of the transformer? Without this allowance you would need to have the combined ampacity of the feeder breakers to be below the secondary conductor rating if a main breaker is not present?

You are missing pieces to the code reference. It is not just qualified personnel.

Exception No. 1:  For installations under single management, where documented safe switching procedures are established and maintained for disconnection, and where the installation is monitored by qualified individuals, the disconnecting means shall be permitted to be located elsewhere on the premises.

So with that being the case does that mean if the facility meets these requirements then a building can have a MLO Switchgear lineup with no disconnecting means within the building that is fed directly from a feeder in another building?

Even still, if there is a primary breaker on this unit substation, this would take care of the disconnecting means. If it wasn't a unit sub, up to 6.

Yes that's a good point, most of the time the unit substation has a primary device (usually a switch) located in the same building.

 

[publicgood]

In order to use MLO with 6 handles or less the combined ampacity of the breakers need to still provide transformer secondary protection and secondary cable protection correct? In a lot of industrial applications this usually is not the case as the combined ampacity of the feeders can exceed these requirements.

If secondary cable protection is indeed required based on 240.21(C)(1). Agree, otherwise.

I was never aware of the allowance you pointed out in 240.92 for supervised industrial locations to allow the primary device to protect the secondary conductors. Would this essentially be confirming that the primary device is below the cable damage curve and has a rating equal to or less than the secondary conductor ampacity reflected to the primary of the transformer? Without this allowance you would need to have the combined ampacity of the feeder breakers to be below the secondary conductor rating if a main breaker is not present?

The key is industrial locations, not just supervised - right. The transformer still needs to be protected according to 240/450, by primary protection only. These rules, and reviewing TCCs, help verify protection of transformer and cables.

In other than supervised industrial, right - combined secondary ratings cannot exceed.

So with that being the case does that mean if the facility meets these requirements then a building can have a MLO Switchgear lineup with no disconnecting means within the building that is fed directly from a feeder in another building?

Yes. In the one case I have done this, the proposed safe switching procedure was presented to and approved by both the local electrical inspector and fire department. Not to say the code requires these approvals, but the installation may be risking AHJ objections if no one asks the question up-front.