GFCI With Additional Coil?

I'm replacing a GFCI in a hot tub. The unit looks like a regular blank face GFCI except for some type of coil on the back that
several wires run through when it is installed.
Here's a picture: http://spapartsnet.com/Electrical-Parts/GFCI/ELGS50A_461_0_4.html#main
Is that coil some type of current transformer? Or what? And what might the purpose be?
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It appears very similar to a Leviton 8895. See spec sheet http://stevenengineering.com/Tech_Support/PDFs/74OEML.pdf . page 4.
It is a current transformer. The GFCI monitors the current flow through the conductors in the hole and trips if they are out of balance. Similar to a regular GFCI except for heavier currents (50 Amps in this case), you route them through the transformer, and you need a relay to control the circuit.
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That's it -- thanks for the reference.
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Paul A wrote:

Nice!
That reply sets an example of how all of us ought to operate: good content, a url *and* a page number at the site so that others would see exactly what you had in mind. Sometimes people give a url but don't identify what they are referring to, so the reader has to hunt around and assume.
Ed
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the
I take it, then, that this gadget controls the load through a contactor/relay?

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In my case, yes.
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