Fuses on Ungrounded DC Systems

It makes everyone feel good.

DC Circuit Protection - Fuse Circuit Protection Practices

If you have a totally floating DC system, the weaker fuse will trip first, so if you have a 10 amp fuse in the negative line and a 30 amp fuse in the positive line, the one in the negative will blow first. The circuit doesn't "know" which is pos or neg... nor does it care. A fuse is fundamentally there to prevent over current burning out the circuitry and causing a fire.
Del

, and ICs are there to protect the fuses!

Do you know my French friends? The standard practice for them, even with a grounded common, is to use a 2 pole enclosed fuse block and fuse the positive and put a solid brass dummy fuse in the common side. When the block is opened, both sides are removed from the system. We've weaned them from doing this on systems sold here in the USA.

Hi all,

Thank you for your replies.

@Del: Under what fault circumstances do you envision the 10 amp fuse to trip first?

I imagine you are referring to a short circuit (connection between positive and negative)?

In that case I would imagine that the smaller fuse (10 amp) trips first and as the current path is interrupted, the second fuse does nothing. It seems to me it would be redundant to have two fuses in this scenario, unless I am missing the point?

To me the question remains: What is the purpose of the fuse on the negative wire in ungrounded systems?

10 is generally considered to be less that 30 . I envision a circuit where the same current is flowing throughout. E.g The load is taking excessive current and it is flowing through both pos and neg leads.

Effectively the 2 fuses are in series and as such the lower rating fuse will blow first.
(Note:- there seems to be some confusion between trips and fuses in this thread)
Del

A1) when the current in the circuit exceeds 10A. <sigh>

"In a floating (ungrounded) DC circuit we install fuses on both the positive and negative wires as both of them have a certain potential from ground".

For a single floating/ungrounded circuit, either DC or AC, neither circuit conductor has any potential to ground.

It is only once a first fault to ground develops that either conductor has a potential difference with respect to ground.

If the fault is on one conductor, then ground will be brought to the same potential as the offending conductor, and the other conductor will be at the prevailing circuit voltage above that. A fuse would not activate.

Only when a ground fault of sufficiently low resistance occurs simultaneously on both conductors of the circuit would a fuse act as a disconnect, and it could be in either conductor. A high resistance fault could remain undetected with ground at some potential to one or both circuit conductors

Upon such a low resistance fault event, a fuse rupture in either circuit conductor would cause current flow to cease, however there would still be a potential difference between ground (now at the same potential as one circuit leg) and the upstream side of the circuit leg on which the fuse has blown. This could present a danger to maintenance staff who may inadvertently come between the two.

Simply fusing both conductors does not remove this danger, but does allow for isolation if they are the only means of doing so.

If there is more than one circuit connected to the same isolated power source, then fusing of both conductors on each circuit becomes advantageous as it is possible that separate ground faults could occur on the opposite conductors of each circuit and, if they happened to be the non-fused ones, then circuit current could be dangerously high.

A circuit breaker acting in both conductors is a far safer method of providing protection as it opens both simultaneously, an RVD would be even better as it detects the first fault.

A 'floating' DC power circuit can attain amazingly high potential with respect to the local 'earth' ground [cgnd]. That potential will be additive with any level within the circuit. Any part of the circuit with the potential to be a safety hazard if shorted to cgnd should be fuzed. If the circuit operated at 28VDC and the potential to cgnd is 1kV there will be little difference between the + and - DC with respect to cgnd. The 'grounded' DC circuit has the - pole tied to cgnd.