What is the difference between B15 lamp cap and B15d lamp cap, B22 lamp cap and B22d lamp cap, E26 lamp cap and E26d lamp cap?

The difference between the B15 and B15d lamp holders is the design of the plug.

The B15 lamp head has only two parallel pins, while the B15d lamp head has three pins, and the middle pin is a grounding pin, which can help the lamp be safely connected to the socket and reduce the risk of leakage and electric shock. B15D lamp holder is commonly used in high-voltage and high-power lamps, such as halogen lamps, xenon lamps, etc. B15 lamp holders are commonly used in low-power, small lamps, such as night lights, flashlights, etc. 

It should be noted that the size, voltage and current ratings of B15 and B15d lamp holders are not exactly the same, so you need to pay attention to their specifications when replacing or purchasing lamp holders.

The main difference between the B22 and B22d lamp holders is the design of the plug. 

The B22 lamp holder has only two parallel contact plates on the plug, while the B22d lamp holder has three contact pieces, and the middle contact is designed to safely connect the lamp holder with ground wire. 

Therefore, B22d lamp holders are safer than B22 lamp holders. In addition, some luminaires require the use of specially designed B22d lamp heads to ensure that they can be connected and work correctly.

The difference between E26 and E26d lamp holders is similar to that of B22 and B22d lamp holders, mainly in the design of the plug. 

The E26 lamp head has only two parallel contact tabs on the plug, while the E26d lamp head has three contact pieces, including an additional ground contact to ensure a safer electrical connection. 

Similar to B22d lamp holders, some luminaires require the use of dedicated E26d lamp holders to ensure they are securely connected and working. Overall, E26d lamp holders are safer and more reliable than E26 lamp holders, especially if grounding is required.