I have the following that ended up

[ChasBean1]

I have the following that ended up in a building.

Two redundant biosafety level 3 (BL-3) exhaust fans are served by a variable frequency drive (VFD). The VFD is equipped with a disconnect and a hand-off-auto switch, configured for interaction with the building automation system that will allow indication for the operators, lead-standby, etc.

Trouble is, the VFD is supplied by a panel at the motor control center that is equipped with overload, another disconnect, and another hand-off-auto switch. The 'auto' position I believe allows for BAS control, but the BAS control will occur at the VFD. So what will the auto switch position at the MCC do?

I recommended to replace the MCC control with a simple disconnect, so that the VFD will serve the H-O-A control and BAS interface. The owner has decided that the current config may be acceptable, so long as it works. I hate to be negative, but I think this installation is poor practice for a BL-3 application. I have more of a mechanical & HVAC background, and I think of it like a VAV box inline with a VAV box, or two flow control functions in series. Not to mention another location from which the fans can be shutdown by accident. Do you have an opinion on this?? Thank you, -CB

 

[rbulsara]

ChasBean:

You are absolutely correct. Get rid of the starter in the MCC and replace it with a disconnect. Use VFD H-O-A switch for BAS controls.

 

[jbartos]

Suggestion: Sometimes, additional control places are governed by an operating procedure. Make sure that you will not be required to reinstate the removed controlled equipment.

 

[advidana]

The owner may think that the building system will still work with your set up. Better be sure also if you mess up the controls will you have to pay for the damages. I would have the owner get an licensed profession electrical engineer to check this set up, then go by his recommendation. That way you are not liable.

Remember, if a electrical person was installing mechanical equipment he would get in trouble and vise versa.

 

[ChasBean1]

I suppose the design intent could have been to interface the building automation system at the motor control center and not the VFD, but I think it was simply a design oversight. Taking the MCC switch to "off" would kill all power to the VFD, which nullifies some VFD advantages like soft starts, etc. Taking the MCC switch to "auto" still leaves me with a question mark.

Thanks for your thoughts. -CB

 

[jbartos]

Suggestion: If the engineering and design have options, they may be presented to the owner for an approval. This way the engineer or designer shifts the responsibility on the owner.

 

[EEIre]

Perhaps it would be a good idea to modify the MCC starter so that you have just the fuses (for short cct protection of the motor) and the contactor without overload for the emergency stop for the motor.

I agree thant the h/o/a and other controls at the MCC should be demolished. The only thing which should open the MCC contactor is an emergency stop.

Regards

Sean