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 Post subject: Isolation transformer = motor contribution?
PostPosted: Wed May 01, 2019 4:38 am 

Joined: Mon Apr 08, 2019 11:25 am
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We have a standard power distribution panel design for a production facility. The system is fed from a 480 V 400 A bus plug with current limiting fuses. The main breaker is set at 350 A, and this feeds into a 220 kVA isolation transformer. On the secondary side are branch circuits feeding remote VFD panels and robot controllers.
In estimating the arc flash current, is it necessary to consider the stored energy in the transformer as a contribution to the available short circuit current?

Thanks in advance for your professional experience.


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 Post subject: Re: Isolation transformer = motor contribution?
PostPosted: Wed May 01, 2019 4:55 am 
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Not sure what you mean by stored energy in the transformers as transformers do not store energy. You calculate the incident energy based on the available fault current, protective device clearing time, enclosure size, working distance and electrode configuration.

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 Post subject: Re: Isolation transformer = motor contribution?
PostPosted: Mon May 06, 2019 1:25 pm 
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Joined: Tue Jan 19, 2010 2:35 pm
Posts: 137
Sorry, do you mean drive isolation or just isolation transformer?
I expect both could, in theory, contribute some current to a system short and possible arc flash. But this assumes you're feeding larger motors using across-the-line motor starters and not VFDs. If that's correct, I'd imagine the winding connected to the motor could see the motor's induced current during a short and possibly induce current in the winding connected to the system. That current would make its way back into the system. But if the isolation transformer is feeding VFD's, then I don't see that the motor would contribute current to a fault. The only concern is if your VFD's have bypasses. If that's so, it's likely the motors may sometimes be operated in the bypass mode. That effectively takes the VFD out of the circuit and would allow a motor's contribution to a fault.


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 Post subject: Re: Isolation transformer = motor contribution?
PostPosted: Tue May 07, 2019 12:43 am 

Joined: Sun Feb 14, 2010 9:28 pm
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Location: Queensland
Isolation transformer is not a motor contribution.
Fault current should be calculated in accordance with IEC 60909 or IEEE 551 as relevant. in both cases various assumptions and simplifications are involved. In general the contributions from cable capacitance and transformer magnetising currents are neglected as stated in the assumptions.


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 Post subject: Re: Isolation transformer = motor contribution?
PostPosted: Mon May 13, 2019 7:46 am 

Joined: Mon Apr 08, 2019 11:25 am
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Thanks for the comments. I misunderstood the issue with the isolation transformer.
The available fault current in the secondary I = E / Z is limited by the impedance of the source plus the impedance of the transformer.
The bolted fault current is 4.3 kA; hence the arcing current is 2.7 kA. This leads to a long clearing time of the primary fuses (1 second)!!!
The Incident Energy is 11 cal/cm2. This exceeds the rating of our everyday PPE.

The transformer secondary conductors are connected to a main panel that maintenance is expected.
It has been recommended to us that we put fuses in a junction box at the transformer to limit the arc flash hazard in the main panel.

Can anyone offer a suggestion whether fast-acting fuses would be appropriate for this application? I am concerned that time-delay fuses won't clear fast enough.


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 Post subject: Re: Isolation transformer = motor contribution?
PostPosted: Mon May 13, 2019 8:02 am 
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What is the secondary voltage?

Dual element time delay RK-5 or RK-1, Bussman FRS-R or FRN-R are probably ok but need more info.

Class J are also probably OK. Also dual element time delay, current limiting.

If you have the ability to model your system to the point you know what the available fault current and incident energy is I would think you'd have the ability to pick different fuses from your software library and see what affect it has on the incident energy.

Also, I'd recommend using a fusible disconnect, not fuses in a junction box.

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 Post subject: Re: Isolation transformer = motor contribution?
PostPosted: Mon May 13, 2019 11:31 am 
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Joined: Tue Jan 19, 2010 2:35 pm
Posts: 137
Regarding recommeding fuses, have a look at Mersen brand A4BQ fuses at the primary or secondary of your transformer. They're very current limiting and might help you reduce the hazards.


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 Post subject: Re: Isolation transformer = motor contribution?
PostPosted: Mon May 13, 2019 11:53 am 
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Ah, re-read your post and see isolation transformer so I assume the secondary is also 480V. That would put you into the FRS-R or as wilhendrix suggested the A4BQ larger or Bussmann equivalent.

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 Post subject: Re: Isolation transformer = motor contribution?
PostPosted: Tue May 14, 2019 5:53 am 
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Location: Spartanburg, South Carolina
If you use a fuse on the transformer secondary, it must coordinate with the branch circuit protection. For a fault on a branch circuit, the branch circuit needs to clear before the fuse, otherwise you will lose service to all remote VFD panels and robot controllers.


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 Post subject: Re: Isolation transformer = motor contribution?
PostPosted: Wed May 22, 2019 7:57 pm 
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Joined: Sat Nov 21, 2009 10:23 pm
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Location: Ohio
Lets try to make this less confusing:

1. The secondary fuses:
A. <= 600 amps would be either Class R or Class J.
B. > 600 amps would be Class L (Mersen A4BQ).

2. The fuse must be sized at a minimum of 125% of the load, in other words, not loaded more than 80%. The wire size must be equal to or greater than the fuse size.

3. As stated a fusible disconnect would be best. Most heavy duty disconnect switches are convertible from Class R to Class J by moving the load side block upward. The Class J fuse is approximately 50% of the the overall length of the Class R.

4. For this type of load you will want to use the non-time-delay type of fuse. I will assume you will use the Class RK1, therefore, this is a Mersen A6K(amp rating)-R.

5. Even with the low fault current the A6K should keep the IE close to or under 1.2 cal/sq cm.

6. In cases where the fuse size exceeds 600 amperes and the requirement is keeping the IE under 1.2 cal/sq cm, you can use the Mersen A50QS +(amp rating). This is a special mounting, however, Boltswitch makes switches through 1600 amperes that will accept the A50QS fuse.

7. Using this scheme, very few process loads will be over 1.2 cal/sq cm. i just did a study today where the circuits were 600 amperes, the IE was close to 50 cal/sq cm with RK5 fuses, the same circuit was approx 2 cal/sq cm with non-delay RK1 fuses (Mersen A6K). This was a circuit with limited fault current.


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