Stabilised Power Line

IIRC, what you want is a ferroresonant transformer, 240 x 120. Sola^® and Constavolt^® are some names that come to mind.

1) Is there variation occurring for a short interval/ varies over the day.?

2) Is it a Motoring Load?

Its just that the input line from the national electricity board is on the low side of the speciication, thats why i need a variac to make sure the stepped down voltage is stable at 120v. If the input voltage increases or decreases, I can appropriately adjust the variac to bring it to 120V. By the way our input line is 240VAC from the substation.

What i need to know is whether i need an isolation transformer to be connected to the output of the variac.

Thanks

Use instead, a servo controlled voltage stabiliser.

Yup, the most stable system with a true sine wave, this things are capable of absorb variations caused by wild load changes, they mimic a zillion KVA power source.

GA

MooMoo - You should not need to isolate the load from the variac. Is your 30 watt tester highly inductive, or does it contain a magnetic ballast? Different Sola and Constavolt products can automatically adjust for the primary voltage changes, or you can use a plain simple variac and manually adjust.

Another thing that needs to happen is a commercially-based moan to the utility supply company about the incoming voltage fluctuations.

Lyn and Tornado are both correct. You could also use simple and possibly less expensive method that will get you clean, stable power. Use a 220 volt battery charger to charge a battery. Connect a quality 120 volt sine-wave inverter to the battery when you want your stable power. This is the same functionality as Lyn's UPS suggestion, but could be less expensive than a dual voltage UPS. Charger - $30 at auto supply Battery - $40-60 at auto supply Quality Sine Wave Inverter for your 30 watt load - $100-150 at electronics supply Now you have a somewhat portable 120 volt supply. You could also consider just the inverter, and run it off of a vehicle battery if one is convenient.

In that case go for any UPS , Chinese equipments takes care of Cost but not quality and European equipments takes care of Quality but not the cost.