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Old 06-11-2011, 10:35 AM   #41
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I'm still lost on dummy fuses. Use normal real fuses on the right side of a motor starter for OL protection, then drive the motor, motor goes to neutral which is un-fused (or at least supposed to be) dummy or otherwise.
The OP is working on a system at a plant with a corner grounded delta system. So he has a grounded phase, which, like a neutral, can't have a fuse in it. The code says where used as motor overload protection however, that that phase can be fused.

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Old 06-11-2011, 10:38 AM   #42
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I'm sorry, I see the distinction but still do not understand the use of so-called "dummy fuses".
Think about it, is it generally a good idea to open any other grounded conductor? The dummy fuse prevents that from happening.
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Old 06-11-2011, 12:05 PM   #43
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The OP is working on a system at a plant with a corner grounded delta system. So he has a grounded phase, which, like a neutral, can't have a fuse in it. The code says where used as motor overload protection however, that that phase can be fused.
Okay, I got that. But if you're using a dummy fuse as overload protection, doesn't that defeat the purpose? Or did I just duck under the point? ):

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Think about it, is it generally a good idea to open any other grounded conductor? The dummy fuse prevents that from happening.
So...why even put a dummy fuse there?!
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Old 06-12-2011, 06:10 AM   #44
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Okay, I got that. But if you're using a dummy fuse as overload protection, doesn't that defeat the purpose? Or did I just duck under the point? ):

More than likely there is another form of o.l. protection in the starter. He has a motor control center with buckets, disconnects, fuses and more than likely thermal over loads in series with the coil on the starters. I am assuming all of this. So the fuses are just being used for short circuit protection. In this case you would use two fuses for the un-grounded phases and a dummy fuse for the grounded phase.

430.36 Fuses In Which Conductor.
Where fuses are used for motor overload protection, a fuse shall be inserted in each ungrounded conductor and also in the grounded conductor if the supply system is 3-wire, 3-phase ac with one conductor grounded.


The second part above would be for a 3 phase corner grounded delta where fuses are used to provide over load protection. If over load protection is already provided it would be safer if not required to install a dummy fuse in the (third phase) grounded conductor.

240.22 Grounded Conductor.
No overcurrent device shall be connected in series with any conductor that is intentionally grounded, unless one of the following two conditions is met:
(1) The overcurrent device opens all conductors of the circuit, including the grounded conductor, and is designed so that no pole can operate independently.
(2) Where required by 430.36 or 430.37 for motor overload protection.


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