For a transformer with primary and secondary ratings of nine amperes or more and no coordinated thermal overload protection, what combination of primary and secondary OCPD percentages is required?

For a transformer with primary and secondary ratings of nine amperes or more and no coordinated thermal overload protection, what combination of primary and secondary OCPD percentages is required?

• A. 250% primary, 125% secondary
• B. 125% primary, 250% secondary
• C. 200% primary, 100% secondary
• D. 300% primary, 150% secondary

Answer: (A)

When a transformer has no coordinated thermal overload protection, overcurrent protection must be arranged to protect the windings without nuisance tripping from normal operation. The primary side is allowed to have a higher multiplier to accommodate inrush and the higher available fault current seen on that side, while the secondary side is protected with a smaller multiplier to guard the secondary winding from overheating without being overly sensitive to the primary’s fault conditions. This combination—primary protection set at 250% of the transformer’s primary rated current and secondary protection set at 125% of the secondary rated current—provides that balance. Therefore, the correct pairing is a primary OCPD at 250% and a secondary OCPD at 125%.

When a transformer has no coordinated thermal overload protection, overcurrent protection must be arranged to protect the windings without nuisance tripping from normal operation. The primary side is allowed to have a higher multiplier to accommodate inrush and the higher available fault current seen on that side, while the secondary side is protected with a smaller multiplier to guard the secondary winding from overheating without being overly sensitive to the primary’s fault conditions. This combination—primary protection set at 250% of the transformer’s primary rated current and secondary protection set at 125% of the secondary rated current—provides that balance. Therefore, the correct pairing is a primary OCPD at 250% and a secondary OCPD at 125%.