I have a rather ancient (1950's??) Ferranti (type L mk2) clamp ammeter with scales up to 1000Amp ac. It presumably is intended only for 50 cycle use. No electronics in it, the jaws are the transformer core, the target wire the primary and a simple moving coil meter with various shunts across the secondary.
Doing proving tests on the new furnace 'body' I was using it as an indicator round one of the four 'power hoses' and was suprised to see indications of 800-900 amps when only running at 25 KW (max is 100 KW). The hoses are electrically paired so presumably something of the order of 1600-1800 amps were flowing if the meter is close to correct. I have never had cause to doubt it at mains frequency.
BUT: today I was running at 2,100 Hz. Now as the frequency rises the transformer will run out of puff as the core is normal transformer lamination stuff but presumably the effect would be a low reading not a high one? If the magnetic circuit can cope with the frequency, the turns ratio stays the same so the reading should be correct ?????
AWEM