ffr 24v system earthed or floating?

[Elliottt]

Is the 24v system earthed to the chassis or is it supposed to foating? If it does share an earth with the 12v system, whereabouts is the main 24v earth lug as I can't seem to find it 

I am getting a reading of 24v from positive to the chassis but that might be an unintentional earth circuit via an appliance


Thanks

[dugite]

I've never even thought about it, but am assuming that the 24v system would be earthed via the 24v alternator? I know that when the 24v system is used from the batteries  for 24v led camping lights and 24v fridges the earth is via the chassis in the usual way.

[Elliottt]

It seems there is a dedicated positive and negative cable ran from the alternator to the distribution box and then to the batteries

if it was earthed at the alternator, one would think that they would just run a positive from the aternator to the distribution box - batteries and just lug the negative terminals to the chassis like the 12v system 

I'll have a proper look tomorrow and report back

all the accessories [fridge, lights, ect I have installed so far i have ran a dedicated positive and negative to the distribution box [via a fuse pane] mainly because they have been short runs and it has been more convenient that way

[Ravvin]

Follow the cable from the negative terminal of your 24v battery bank and see where it goes. If it bolts to the chassis, there's your answer. If it bolts into some sort of distribution block, there should be places for all the negative wires from the radio gear to connect to, although I see on this forum that the radio vehicles used a distribution board with fuses and things to protect the gear, although the earths in the box could all lead to the chassis as well.
Modern 12v alternators use a direct connection to the engine as the earth side, which is why you only see a single heavy cable coming from the alternator and going to the battery, often with a smaller wire connected back through the ALT light as a field exciter and sometimes another leading to the tacho.
I think the 24v units had a big bundle of wires coming out of it, so may earth through one of them or still use the engine/chassis system.

[Dervish]

In an FFR you have two 24V battery banks, so even the presence of earth cables back to the alternator won't tell you whether it is floating or not.

Whenever there are batteries in parallel, it's important that the effective resistance of both battery banks are the same. The resistance of a battery is minuscule, so any difference between the two banks will drastically reduce the charging of one bank. I have a standalone power system design background; wherever we NEEDED to have batteries in parallel - something we avoided like the plague - each bank would have its batteries tested and matched to the other and the cables run to each bank would be the same gauge and length (the excess length for the shorter cable run would be coiled). Even a difference in the length of cable to each bank could introduce a resistance that would throw the charging out of whack. One bank would have a float resistance of 1.7 ohms and the other 2.1, for example. Under those conditions, one bank would charge preferentially while the other would slowly deteriorate - eventually to the point where it would suck the life out of the healthy bank.

I've read about people having problems losing batteries in FFRs and I bet mismatched resistances are the cause. I would personally remove one bank of batteries from the system and avoid the headache altogether. As for floating or not, I don't know - but remove your appliances, measure the voltage again and you'll soon find out.

[Elliottt]

The lhs and rhs batterie banks are just bridged together in parallel via about 1 meters of cabe with the main termination from the distribution box at the lhs I've already discarded the rhs battery bank and disconnected the bridging inks as i only need [and afford] 2 batteries

I had a bit of a closer look this morning and i still can't find an obvious earth point

Positive comes from the alternator to the dbox, where it goes to the line side of the 100 amp circuit breaker, then via the shunt via a fuse to the lhs battery bank

Negative comes from the alternator to the dbox, where it terminates to a [negative] bar, the negatives for the batteries and appliances a lug off the bar

[BrookyLR]

The wiring diagram in EMEI G122 suggests that a link to earth can be inserted within the EDB "if coupling to vehicle earth is required". However, I cannot see where this link is supposed to be inserted and where this would go to earth if the link was inserted (I have not opened up the EDB itself).

From this phrase, I am deducing that the 24V system earthing is normally only via the 28V alternator. I certainly cannot find another earthing point for the radio circuits.