Battery Charger

The risk to personal safety persists for exactly the same time.

Why an earth fault should persist for a long time with the availability of modern residual current devices and modern wiring regulations is abstruse.

There is "no effect" until a second fault occurs, then you will wish that you had performed proper protective and maintenance practices to alert and correct the fault.

It depends if one pole is earthed!

If you have a situation where a battery and its connected loads are well-insulated & not earthed at all, except by an earth fault detection relay, then I have known a plant sitting for years [with an earth fault alarm (One pole 40V to earth, compare 65V fault-free) - disconnected from the annunciator!] until the battery cells were washed externally to remove the traces of damp electrolyte which were causing earth leakage (more on one pole than the other).

The relay connected each pole of the 110V battery to earth by a 50,000 ohm resistor. The relay alarmed if the voltages to earth were unbalanced by more than a preset amount. Voltages of battery poles normally +/- 65V to earth (battery float charge 130V) with no leakage - consequently possible current, about 2 mA, into earth fault is harmless to most equipment. Compared to a "solid-earthed" battery, there is also a personnel safety advantage.

There is also a system (e.g. telephone supplies) where the positive of the battery is biased negative above earth by the earth fault detection relay integral DC source. This was because electrolytic corrosion due to leakage currents is worse at the positive end. Since the telephone end may be the fine wire of a coil, it is far more vulnerable to corrosion than a large earthed metal case. By biasing everything in the system negative a version of "cathodic corrosion protection" was achieved. There is also the advantage that everything in the system has a definite voltage to earth, to allow earth fault detection - the centre tapped system has the possibility that a point of the load which was at zero potential could be earthed without causing an alarm.

The current permitted by the earthing resistors is selected to be too small to spuriously operate or hold-on a relay or PLC input, but enough to give warning of a problem. Also the resistance is low enough to avoid an alarm due to small currents which are sure to exist in a real plant.

67model

dear sir

thank for your replay

whats the different between earthed battery charger & non earthed battery charger,and are there any

difference between positive earthed and negative earthed

best regards

First question...apart from the obvious earthed or not earthed.

Obviously a lot of batteries (like in portable radios) are just not earthed. But when you connect a mains powered charger you bring in the voltage to earth due to the AC secondary windings and their capacitance to earth or even, if the transformer is double insulated (rather than having an earthed foil between primary and secondary), higher voltages due to primary-secondary capacitance.

So it is usually a good idea to "earth" through some resistance and or capacitance to stop high voltage to earth.

I explained that positive earth has arguments in its favour. Nontheless, negative earth systems can work perfectly well - motor cars used to be positive to chassis but negative to chassis seems universal now.

Negative-earthed vehicles don't suffer from body and chassis corrosion nearly as much as positive-earth ones.

Monsieur Crabtree - do you know of an industry report comparing pos & neg earth cars? I do have a recollection of, long ago, change of a truck model pos to neg earth without any other change reporting reduced electrical faults.

It is consistent with the positive end suffering worse corrosion than negative.

In the days of occupied France and "Dee-Go-al" there was much use of chassis as electrical connection, but not nowadays.

Since the warranty on a car chassis is much longer than on the electrics, I guess Renault etc would rather reduce chassis corrosion. On a big generating set preserving the controls is more important!