SELV Fusing

I'm not sure you are looking at the right problem Marty. The only problem you can get with SELV is surely overload, nothing to do with Earth faults. A single fuse will protect against excess current, if you want anything else you must consider some mechanism of Earth fault detection. SELV is completely isolated from Earth, 414.4.1. Do you mean PELV?

I'm not sure you are looking at the right problem Marty. The only problem you can get with SELV is surely overload, nothing to do with Earth faults. A single fuse will protect against excess current, if you want anything else you must consider some mechanism of Earth fault detection. SELV is completely isolated from Earth, 414.4.1. Do you mean PELV?

Hi David,

I was not really thinking of any particular fault type, more the principal of fusing both legs of the ELV.

Just found the source of the original debate…

“One problem with a PELV system is that faults elsewhere in the installation may induce voltages on the entire system via its protective earth conductor.

The advantage of an SELV circuit is that there is no earth return path which might occur via the human body. A disadvantage is that an over-current protection device must be installed in each leg of each ELV circuit and sub-circuit.”

source David W Otterson

journals.sagepub.com/.../0020294017701880

Martyn said:

One problem with a PELV system is that faults elsewhere in the installation may induce voltages on the entire system via its protective earth conductor.

Not sure how an OCPD will protect against, that, but the advantage of PELV for a control system, is that faults operate protective devices, and, if the controls are arranged so that "ground" or "off" is a default safe state, the fault is both detected, and leaves the system in a safe state.

Another advantage of PELV is that static charges are eliminated, and , provided the current-carrying conductors in the system are separated from PE (a TN-S arrangement), PELV can also be an effective way of dealing with certain electromagnetic compatibility and overvoltage phenomena.

Strictly, the voltages impressed on a PELV system are controlled in the same way as main protective bonding, and that is why standards such as BS EN 60204-1 have such strict requirements for the protective bonding of the machinery covered by the standard.

However, SELV also has its advantages under other conditions, and equally  it may be advantageous to separate some signals from PE.

Martyn said:

The advantage of an SELV circuit is that there is no earth return path which might occur via the human body. A disadvantage is that an over-current protection device must be installed in each leg of each ELV circuit and sub-circuit.

PELV has the same voltage source considerations as SELV, so I'm not quire sure about the "earth return path" point there. Yes, if there is a voltage rise on PE resulting from a fault in another circuit, that voltage will be impressed on the PELV system, as on the whole PE system, but the relevant systems or equipment should be designed so that's not an issue?

As per my earlier reply, I don't think it's necessary to provide an OCPD in every pole on an SELV circuit according to BS 7671, certainly not in a 2-wire SELV circuit, for the reasons provided, so unless the previous discussion expanded on why it was thought this was necessary. I'm not saying other standards don't have the requirement.