LONDON, July 31 (Reuters) - Copper trading on Comex was

halted on Wednesday, the exchange said, reflecting the magnitude

of the price plunge of more than 20% in an hour after the U.S.

announced its decision not to apply tariffs on imports of the

metal.

Copper traded on Comex, part of the CME Group ( CME )

, crashed 22% to $4.5030 a lb or $9,927 a metric ton

after the announcement, sparking a rapid unwinding of long

positions that triggered the circuit breaker.

Trade stopped for two minutes between 1314 and 1316 Central

Time (1814 and 1816 GMT).

This was the second time in less than a month that circuit

breakers were triggered in the copper market. On July 8, prices

spiked nearly 18% after President Donald Trump said he would

announce a 50% tariff on copper imports.

"The market got ahead of itself," said one copper trader.

"Once it became clear that refined copper was exempt, the

balloon deflated very quickly."

The United States said on Wednesday 50% tariffs would apply

to copper pipes and wiring from August 1, but the restrictions

left out copper materials such as ores, concentrates, and

cathodes.

"For metals markets, the range in which prices can move is

reset continuously on a rolling 60-minute lookback window. If

markets move +/- 10% in that time, a 2-minute halt is

initiated," CME Group ( CME ) said in a written comment.

"We can confirm that a dynamic circuit breaker for copper

futures and options was triggered yesterday at approximately

1:14 p.m. CT, when the market halted and transitioned into a

two-minute pre-open state."

Comex copper traded on Thursday around $9,600 a ton and its

premium over the London Metal Exchange benchmark contract

has narrowed to around $10 a ton compared with records

around $3,000 a ton earlier this month.

That premium had attracted copper from around the

world since the tariffs on metal were mooted by Trump in

February, much of it stored in LME-approved warehouses.