fault withstand level in switchboard

May be

1)The Z value of the transformer low

2)Transformer is nearer to the source

3) Transformers operating in parallel

Basically if the Impedance is low the fault current will be high then the switch boards has to be designed for to withstand the fault current for a specified time.

Ther are some elements missing on your question, certainly the distribution system has been changed, transformer power and caracteristics and some users also addition of big motors?

Question shall be more precise.

<...Question shall be more precise....>

The CR4 readership may well be looking forward to that state of affairs with excitement.

its actually, earlier customer was asking for 50 kA 1 sec in switchboard. now they ask for 85 kA 3 sec. what is the reason behind?

It seems there is drastic difference in fault level requirement.

May be the customer operating the transformers in parallel.You may request the customer to furnish details.

If the customer is considering an increase in the size of the transformer, the fault current will go up.

Size is not the main issue. A lowered impedance will allow greater fault currents for that brief moment when trouble occurs before the protection is activated. The rating states how much fault current the device or panel should be able to withstand momentarily without destructing. The impedence is affected mainly by sheer closeness of the supply transformer and by parallelling transformers to add available amperage. Also addition of capacitor banks can have a direct effect on the impedence.