chassisfusesgroundsafety
I am working on a vehicle with batteries, but i am not quite sure if my grounding solution/design is correct. It is supplied with two 12VDC batteries in series, connected to a DC-DC (24VDC-24VDC) powersupply. Basically i always ground the 0V to the chassis, and use a 2-pole MCB after the batteries and 1-pole to devices. Is this a bad design?
Warning
Equipment chassis is usually grounded to wiring ground via earth lead on power plug where provided.
In any sane regulatory environment, appliances without an earth lead are required by law to be "double insulated" so that internal metal parts are not touch accessible during operation. (Note: Double insulation nowadays seldom involves two layers of insulation - it is more a state of mind in manufacture and testing that leads to touch safe equipment. Usually :-). )
While a chassis should be "touch safe", never assume it is, because:
There are non-sane regulatory environments,
or ones where the authorities enforce the rules so little that they may be ignored and
It is excessively common for people to do silly and dangerous things with mains wiring, despite being aware that people are reported to have died due to such things.
They say:
Class I
Class II - See also: double switching (and double insulated)
A Class II or double insulated electrical appliance is one which has been designed in such a way that it does not require a safety connection to electrical earth (US: ground).
The basic requirement is that no single failure can result in dangerous voltage becoming exposed so that it might cause an electric shock and that this is achieved without relying on an earthed metal casing. This is usually achieved at least in part by having two layers of insulating material surrounding live parts or by using reinforced insulation.
In Europe, a double insulated appliance must be labelled Class II, double insulated, or bear the double insulation symbol (a square inside another square)..
Class III
A Class III appliance is designed to be supplied from a separated/safety extra-low voltage (SELV) power source. The voltage from a SELV supply is low enough that under normal conditions a person can safely come into contact with it without risk of electrical shock.
The extra safety features built into Class I and Class II appliances are therefore not required. For medical devices compliance with Class III is not considered sufficient protection.
Here is a simplified version of AC power lines:
simulate this circuit – Schematic created using CircuitLab
The two lines at lower right represent you holding the neutral. Since the neutral is connected to ground elsewhere, as you agree you only feel a small voltage. Note that the load is fairly low resistance, so it can produce a lot of power.
Now let's break the neutral
Current will now flow through the upper hot wire, through the load, through you and then the ground in order to get back to the transformer neutral. And since the load has a low resistance, you are the big resistor in the circuit, and you will take most of the voltage.
Best Answer
I've taken your diagram and added two circles and a few lines: -
Firstly, the motors at the bottom - I don't think you should label the two wires to each motor as 24V and 0V - they are likely to be PWM outputs from the controller that form a H bridge and, if i'm correct there will be no current to ground or real 0V. I say no-current but the chassis will pick-up all sorts of interference from the heavy current fed to the motors and avoiding the chassis is best policy other than for a single connction (if necessary).
I've also drawn a circle around the controller and plc to indicate that there will likely be some control lines between the two or else how are you going to regulate motor speed? The type of connection needs establishing i.e. is it differential signalling? Do the signals share a local 0V? Can it be opto-coupled (least likely to give problems)?
When it comes to wiring, you have shown a bus type arrangement and this is OK for a circuit but you have to feed the 24V batteries (via over-current protection) directly to the controller. You can tee-off the lighter loads (dc-to-dc converter) directly from the battery. This means you have to star-point two 24V and two 0V conections at your battery terminals to avoid motor current giving problems to the converter etc..
Keep the wires from the battery to the motor controller as short as possible is a must.
You also, on your circuit have two over-current devices in the path from your battery to motor controller - I would avoid this - use F3 directly for the motor controller and I believe F4 is kind of redundant because it is protected by F2 in front of the dc-to-dc convertor. F1 I think would be redundant but if you have other circuits (not shown) then use an adequately rated device for those circuits.