Fuse protected transformer feeds customer's arc resistant switchgear. Switchgear's arcing current rating is well in excess of the calculated value, but the rated duration is 0.5 s in accord with IEEE C37.20.7. Customer wants primary protection that will not exceed 0.5 s for the calculated arcing fault. Is this a common consideration when applying protection? Do you consider it? Thanks.

Yes, this can be done. There are electronic fuses available. There are several types from different manufactures. Some of these fuses have an external trip unit or relay that causes the fuse to blow.

Sure, anything is possible. I'm more interested whether this is a common consideration. Most equipment and conductors follow an (I^2)t type of damage curve that allows time-overcurrent coordination. Now we've thrown a flat definite time damage curve into the mix, which sounds like it might be a problem. Is this really how this gear is implemented?

Confession: We recently added arc resistance to our specification for medium voltage outdoor breakers. Seemed like a bit of added safety at very little cost, and one manufacturer will only offer this style going forward. It never occurred to me to change the upstream protection settings for these installations.

Take a look at the Mersen HMH and XMH fuses. There are are 15 kV fuses for use up to a 2000 kva transformer, they typically bring the secondary incident energy down under 10 cal/sq cm.

You will have to add the XMH fuse to your library since it is new.

Thanks. I'm still less interested in how to do this. I'm much more interested in if it is done. Perhaps I should attempt a survey question.

It is my impression that much of the AR gear installed is improperly protected exactly for the reason you state. The gear has an incoming section protected by a remote device, often a fuse on the higher V side of a transformer. Unluckily, MV fuses are not very good at LV AF protection devices. So if your question is if it happens, the answer is yes, it happens. If your question is "should it happen" my answer would be no it should not happen as it, essentially, renders the so called AR rating somewhat, if not completely, moot.... there are related issues around ties where an Arc could spread to both sides of an open tie creating a double fault...

Careful selection of the fuse, assuming you can dial it down, using electronic fuses or some other technique may improve the situation. However, if the gear has 0.5 second rating, then the protective device MUST protect within that time at expected arcing current values or else the AR rating is not usable.

Generally, you will need a 3 cycle primary CB and well engineered relay protection to protect the substation transformer secondary upt to a through the LV main to achieve selective and fast protection. Multiple relay schemes are possible

There is no I2t extrapolation you can make. The gear is not tested that way and it would not make sense.

. Thanks for the comments. I'm not sure the added protection is moot. With a choice of the arc blowing open the doors with a worker standing in front versus the arc being directed up and away, I think I'd opt the the latter. Half a second would allow some increase in separation, and the worker is wearing clothing rated for the exposure anyway. A definite time element set faster than 0.5 s upstream won't allow much coordination with downline equipment even with a three cycle breaker.

Agree, definitely better to employ AR gear than non-AR. However, if an arc happens and the protection is slower than what the AR rating requires what happens is unpredictable. So standing in front of the gear with lower lower level PPE may incurr some residual risk.

As far as protection clearing time for substation protection 0.5S seems slow by modern protection standards. Optimized protection around the substation transformer (secondary substation) can be designed to clear in under 100ms with no sacrifice in selectivity, particularly if you use 87T or advanced ZSI techniques.

If you use the fast Mersen fuses make sure you account for all delays associated with interposing devices and communications.