Ex e MV motors Vs Ex e LV motors - Zone-1

[NickParker]

Ex e Low Voltage motors are allowed in Zone-1, but Ex e medium voltage motors are not allowed in Zone-1. Can you please explain the reason?

 

[Kiribanda]

To my knowledge Ex-e concept/ design is approved for Zone 1 & Div 1 operation.
Do you have any written statement or any standard showing that otherwise?

 

[Gr8blu]

Ex e refers to "increased safety due to a lack of arcs, sparks, or sufficiently hot surfaces". In some medium voltage machines ("medium" being anywhere from 1 to 10 kV), the air gap may be small enough that during specific transient events, a spark will develop between stator and rotor steel which could - in a Zone 1 environment - ignite a flammable gas mixture inside the machine enclosure. This does not occur with a lower (i.e. < 1 kV) rating because there is insufficient energy to cross the gap.

Note that both altitude (due to decreased gas density) and humidity - and the presence of conductive contaminants - may all affect the voltage level at which the phenomenon occurs.

As Kiribanda pointed out - there are a lot of machines out there operating in Zone 1 environments that utilize voltages up to 13.8 kV. Enclosures may differ from their low voltage relatives, and maybe different precautions in terms of preventing the ingress of combustible gas.

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[Kiribanda]

Also the HP/ kW ratings of Ex e motors are little lower than a Ex n motors due to the inherent limitations of Ex e design concept.
But still it is good for Zone 1 applications.

 

[edison123]

All machines are well insulated for their rated voltage. I don't see how the MV machines could create a spark across the air gap. PD across the stator slot walls (in poorly made machines may be) but definitely not across the air gap.

Muthu

http://www.edison.co.in

 

[wcaseyharman]

In some MV generators we’ve seen surface PD between end arms in the end winding region due to inadequate “vent spacing”, I would assume that would not be ideal for explosive environments.