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Dry type XMFR - 7.5kV rated cable in a 25kV system?

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  1. ekemp2 is offline Junior Member
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    Dry type XMFR - 7.5kV rated cable in a 25kV system?

    Hello all,

    I recently was testing a 750kVA dry type distribution transformer for regular maintenance. The transformer system voltages are 25kV - 600/347V. The jumper cables from the bus to the primary windings is rated for 7.5kV. The cable is zap strapped to glastic barriers from the bus terminations all the way to the windings.

    I am wondering if there is a standard that allows for cable insulation deration. Is it ok that the cable is only rated for 7.5kV in a 25kV system. The transformer has been running for a number of years without an issue.

    Thank you for your input.

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    Quote Originally Posted by ekemp2 View Post
    Hello all,

    I recently was testing a 750kVA dry type distribution transformer for regular maintenance. The transformer system voltages are 25kV - 600/347V. The jumper cables from the bus to the primary windings is rated for 7.5kV. The cable is zap strapped to glastic barriers from the bus terminations all the way to the windings.

    I am wondering if there is a standard that allows for cable insulation deration. Is it ok that the cable is only rated for 7.5kV in a 25kV system. The transformer has been running for a number of years without an issue.

    Thank you for your input.

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    Would you have even given it a thought if it was was bare copper tubing or bus?

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  5. SecondGen's Avatar
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    Quote Originally Posted by ekemp2 View Post
    Hello all,
    Is it ok that the cable is only rated for 7.5kV in a 25kV system. The transformer has been running for a number of years without an issue.
    Seems fine because that glastic barrier is working to reduce the electrical stress on the cable. There doesn't appear to be any grounded components remotely close to the cable insulation so there is little cause for concern.

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    Quote Originally Posted by ekemp2 View Post
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    On second look of this picture, there does appear to maybe be some tracking at the top where the cable bends over. I would inspect that area, it could just be where the factory cut the glastic.

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    Kalbi_Rob is offline Experienced Member Pro Subscriber
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    Quote Originally Posted by ekemp2 View Post
    Hello all,

    I recently was testing a 750kVA dry type distribution transformer for regular maintenance. The transformer system voltages are 25kV - 600/347V. The jumper cables from the bus to the primary windings is rated for 7.5kV. The cable is zap strapped to glastic barriers from the bus terminations all the way to the windings.

    I am wondering if there is a standard that allows for cable insulation deration. Is it ok that the cable is only rated for 7.5kV in a 25kV system. The transformer has been running for a number of years without an issue.

    Thank you for your input.
    As for the voltage rating of the transformer, that seems very low (agreed). Transformer Manufacturers have specifications for the specific cabling installed internal to their transformers. I would contact the manufacturer for the specifications on this transformer. Taking a pre-cursory glance at the manufacturer, it appears they only deal with transformers up to 4160V, which would mean this was probably a special order. That being said, one of the installers may have just grabbed cable that was available vs. the cable called out per specification. That would then be a Manufacturer defect and the customer could require the manufacturer to cover the cost.

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  11. Comx10 is offline Junior Member
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    Quote Originally Posted by Kalbi_Rob View Post
    As for the voltage rating of the transformer, that seems very low (agreed). Transformer Manufacturers have specifications for the specific cabling installed internal to their transformers. I would contact the manufacturer for the specifications on this transformer. Taking a pre-cursory glance at the manufacturer, it appears they only deal with transformers up to 4160V, which would mean this was probably a special order. That being said, one of the installers may have just grabbed cable that was available vs. the cable called out per specification. That would then be a Manufacturer defect and the customer could require the manufacturer to cover the cost.
    The cable coming into the XFMR is normally installed by the customer or contractor from what I have seen, so I would think it is the customers cost unless otherwise specified by the manufacture and proven otherwise. I would certainly think the cable insulation should be looked into and the life of the cable is being shortened dramatically and lead to possible major issues down the road. I suppose it would depend on the insulation rating and amperage actually being used as well. Certainly start with the manufacturer and inform the customer of the find even though it sounds like it was out of scope, certainly seems suspicious.

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    Quote Originally Posted by Comx10 View Post
    The cable coming into the XFMR is normally installed by the customer or contractor from what I have seen, so I would think it is the customers cost unless otherwise specified by the manufacture and proven otherwise. I would certainly think the cable insulation should be looked into and the life of the cable is being shortened dramatically and lead to possible major issues down the road. I suppose it would depend on the insulation rating and amperage actually being used as well. Certainly start with the manufacturer and inform the customer of the find even though it sounds like it was out of scope, certainly seems suspicious.
    From the pictures it looks like those cables are within the transformer, i.e. cabling from taps to output of the transformer. Those should be purview of the manufacturer. Those cables typically land on a bus connector where the customer connects to. The cables from a MV Air switch or etc. to the transformer are the customer cabling.

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  15. bec51392 is offline NETA Level III Pro Subscriber
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    This is an age old argument.
    I agree that cable rating seems low. It very well may be the wrong cable. In my limited experience installing these dry types, that cable comes partially installed or even hand-coiled inside for the final termination to be made once the trans is set. Someone may have installed the wrong cable or the wrong cable was provided in the parts package. But I wouldn't raise the red flag yet.

    Manufacturers get much more wiggle room on things then we do. Once it is a "listed" assembly they get to follow different rules. In this case its CSA listed.

    For a super rough comparison. NEC says if its a 20amp circuit, 20 amp breaker ect that all wire in the circuit should be at least #12 AWG... but many light fixtures have #18 or smaller wire coming out to make the term to the feeder.
    So do you change the factory installed wire to #12? No. Its part of a listed assembly being used as designed so everything is good.

    I would reach out to the manufacturer with your question and see what they say. Might be a good catch, might be nothing.

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