For welding purposes, the secondary transformer used should be capable of carrying low voltage and high current. This is because welding requires a high amount of current to generate the necessary heat for melting and joining metals. Let's break down the answer in detail:

Benefits of Low Voltage:
1. Safety: Low voltage is safer for the welder as it reduces the risk of electrical shock. Welding involves working with high temperatures and sparks, and a lower voltage helps minimize the potential for accidents.

2. Control: Low voltage allows for better control over the welding process. It enables the welder to adjust the heat input and penetration depth more precisely, resulting in cleaner and more accurate welds.

Importance of High Current:
1. Heat Generation: Welding involves melting and fusing metals together. To achieve this, a significant amount of heat needs to be generated. High current provides the necessary heat energy to melt the metals and create a strong bond.

2. Penetration: High current allows for deeper penetration into the base metal, ensuring a strong and durable weld joint. It helps to create a robust weld that can withstand stress and load.

3. Efficiency: High current enables faster welding speeds, increasing productivity. It reduces the time required to complete a weld, making the process more efficient.

Reasons for not using High Voltage:
1. Safety: High voltage can pose a greater risk of electrical shock to the welder. It is important to prioritize safety in welding operations, and using low voltage helps minimize this risk.

2. Control: High voltage can make it more challenging to control the welding process. It may result in excessive heat input, leading to distortion, burn-through, or other welding defects.

Conclusion:
In welding, the secondary transformer should be capable of carrying low voltage and high current. This combination ensures safety, better control, efficient heat generation, deeper penetration, and increased productivity. Welders must understand the importance of using the appropriate voltage and current levels to achieve high-quality welds and maintain a safe working environment.