5kVA 480V to 120V single phase transformer blowing fuses, and shouldn't be.

[rambojoe]

I've spent probably 1,000 hours over the past two years drawing up schematics for a customer's machines that don't have a scrap of documentation. It drives me up the wall how common it is in the industry to have no electrical docs.

That is indicative of fuse changers, not good electrical facility workers who care about the companies tools...

 

[gar]

230429-1531 EDT

sparkie1:

How something works is defined by physics, not by code.

Code has been written to provide guidance on how to work with something in a safe manner. Generally code does a good job of this function, but code just does not define how something works.

You have to understand how something works, and then you can see how code applies.

The rating on a fuse or circuit breaker is generally defined by where it trips after some long duration of time. Generally you don't know a lot about a fuse for shorter times and higher currents. There are some broad definitions, for example a slow blow fuse, or different trime time curves for different breakers.

The process and how it works controls how you select a protection device.

Because you have a very low load on a transformer secondary does not mean you can have an input breaker of a low rating. That input protector has to be selected for evey possible normal input current to the transformer.

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[petersonra]

I've spent probably 1,000 hours over the past two years drawing up schematics for a customer's machines that don't have a scrap of documentation. It drives me up the wall how common it is in the industry to have no electrical docs.

I have gotten used to it. usually one can figure it out. What aggravates me is how many people do not have reliable backups of PLC and HMI programs. When the PLC eventually dies they are really screwed without a backup program.

 

[topgone]

Thanks for the help on this everyone. As far as the next size up rule, it's a bit of a tricky one. From Table 450.5(B), the primary can't use the next size up rule, but the secondary can. As an aside, does anyone know what this "coordinated thermal overload" is? I've never actually looked into it, and a quick google just gives me a bunch of code cites rather than the definition.

It's basically plotting TCCs of all overloads that you are using for a fault on the system. The desired coordination is for the protection nearer to the faulted section should trip ahead of the protection higher upstreams --> no overlapping of the curves.

 

[JoeStillman]

Coordinated thermal overload is common in very large power transformers. Here's a link to a scholarly paper... https://www.erlphase.com/downloads/papers/GaTech_ThermalOverloadProtectionofTfmrs.pdf

The protection that is of interest to this paper is the ANSI 49 Thermal Overload function
available in modern numerical transformer protection relays. This function, depending on the
specific implementation in the relay, uses some combination of measured current, ambient
temperature, and transformer oil temperature to detect the presence of an over-temperature
condition. The function can then alarm the presence of an over-temperature condition, remove
load from the transformer, or trip the transformer off-line.