Overcurrent protection for fractional HP motors in hazardous areas 3

[AlexWNZ]

Hi All,

I'm just querying a recent design I received from an engineering house which is for a 0.75kW Ex Na 400V star connected motor.

The reason I am concerned is that the design does not have a dedicated overload relay. -All that is present is 3x4A fuses and a contactor (no protection against single phasing, and overload functionality with 4A fuses is set at the equivalent of 2.7KVA??).

I've challenged this and they have said its ok but this seems wrong for a Hazardous area rated motor when I am used to seeing hazardous area rated motors with a service factor of 1 and quite strict thermal overload controls (need to prevent surface temps exceeding limits as we are a T3 - 200degree max surface temp site).

Or is this actually permissable? If so can you explain as I'm a bit lost?

Thanks!

 

[AlexWNZ]

Just to clarify, the motor does have a thermistor but it is not connected in the design

 

[electricpete]

Just to clarify, the motor does have a thermistor but it is not connected in the design

That doesn't seem right to me. I don't know a lot about the requirements, but all of our hazardous environment motors trip on motor temperature (Kixon type switch).

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(2B)+(2B)' ?

 

[jraef]

Seems to me that a thermal snap switch (aka Klixon) would be contraindicated in a classified environment because the switch would be in the motor power circuit and thereby a source of ignition. That’s why the direct thermal protection of classified motors is usually done with embedded thermistors wired to a thermistor sensing relay or circuit.

AlexWNZ,
I can’t speak to your local codes, but here in the US is it acceptable to use line fuses as both the Short Circuit Protective Device and the Over Load Protection, provided that the fuse selection falls within the relatively narrow bandwidth of acceptability as the OL device while simultaneously not exceeding the limits of the SCPD while allowing for inrush without nuisance clearing. That is very difficult to accomplish, but not impossible, especially on fractional motors.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Compositepro]

A Klixon in the motor windings is inside the explosion-proof motor casing so that acceptable.

 

[ScottyUK]

CompositePro - you're way off the mark. Explosion-proof is a North American term, and it's broadly equivalent to Ex 'd' Flameproof under the EN 60079 series of standards. The motor in question is an Ex 'n' type which is broadly equivalent to a Class 2, Div 2 machine in your country. It is totally different to an Ex 'd' or XP machine.

For the OP, there's nothing in the 2005 version of EN 60079-15 which says that overload protection on an Ex 'n' machine differs from that for an equivalent safe area machine. I don't think there were any requirements imposed in more recent versions, but I'm not in work at the moment and can't easily check.