220VDC supply grounding

We use a PLC to control a 220VDC supply (and a lot of amps)to an electromagnet. The PLC controls big contactors which when 'on' is selected, put the full 220VDC over the magnet. When off is selected the contactors switch in a dump resisitor and connect both sides of the magnet to the 0VDC rail.

Now it is this aspect which worries me. If someone unplugs the magnet when it is 'off', and touches the 0 Volt rail could they get a shock? At the moment I do not think the electrical techs ground the DC supply 0V rail. To me that means that 0V could be floating and still give a shock. The DC supply is provided by a transformer rectifier.


Does anyone have any ideas about this? Big electrical stuff is not really my area, but having seen the circuit diagram that the PLC is to control I got a bit worried.

If it is un-grounded then it can float and give a shock. If you want to ground it, check for earth leakage circuit breakers first.

benjo said:

We use a PLC to control a 220VDC supply (and a lot of amps)to an electromagnet. The PLC controls big contactors which when 'on' is selected, put the full 220VDC over the magnet. When off is selected the contactors switch in a dump resisitor and connect both sides of the magnet to the 0VDC rail.

Now it is this aspect which worries me. If someone unplugs the magnet when it is 'off', and touches the 0 Volt rail could they get a shock? At the moment I do not think the electrical techs ground the DC supply 0V rail. To me that means that 0V could be floating and still give a shock. The DC supply is provided by a transformer rectifier.


Does anyone have any ideas about this? Big electrical stuff is not really my area, but having seen the circuit diagram that the PLC is to control I got a bit worried.

Click to expand...

can you sketch out a circuit ?

L are the lift contactors, D the drop contactor.

Am I right to think that only one side of de 220VDC is controlled (switched on and off) and that the other side is connected directly to the rectifier and further to the magnet?

If so, all depends on the type of transformer being used:
- If it's a step down transformer (the secondary winding is part of the primary) the 0V rail will always be live. There's a direct link between the secondary (thus the 220VDC) and the mains power lines.
- If you're using an isolated transformer (having a separate primary and secondary winding) and the secondary winding is NOT grounded it will be safe to touch the secondary or the DC supply (only one side at a time of course). However, due to the slight capacitive effect in transformers currents in the range of µA's (so not really life treatening) can flow if the secondary winding is not grounded.

But actually the main issue here should be the fact that the 0V rail can be touched when the magnet is unplugged; This is not a normal situation. Any type of plug should be at least IP20, also referred to as 'finger save'. Even in a cabinet all rails and naked connections must be guarded, unless a mains switch on the outside of the cabinet prevents the cabinet from being opened while in the ON position. Maybe I'm missing something but you should focus on prevention (to touch live conductors) first.

Hopefully I was of some assistance.

Lift is a direct connection to the coils. Drop reverses the polarity, and powers through R1. Whenever both Lift and Drop are off, the energy is discharged through R3.
Nothing in this part of the circuit shows a shock hazard.
Check the power supply. I agree will everything from Sparkz (above).

The transformer is an isolated transformer with no earthing on the secondary side.

The plugs are awful. That is known and I believe they are trying to do something about it. However due to the nature of the operation the current design is necessary and there is considerable difficulty in replacing them.

Hi Benjo,

Still, I have to repeat: Protection against direct contact is only possible if all materials have a minimum IP factor and they should always be in top condition. Anyhow, it is allowed to leave the secondary ungrounded.

However, protection against indirect contact (due to an isolation failure) should be installed by means of an isolation guarding device. It's always possible that the isolation of any conductor on the secondary side gets damaged, but this should not directly lead to a hazardous situation. So it's not necessary to interrupt the mains on a single isolation failure; a warning signal like a buzzer or a control lamp is enough. The guarding device checks the impedance between the circuit and the ground. Whenever this impedance drops below a critical point the device will signal it and you will have to reset it manually. Any kind of isolation error, whether it's in the transformer or even a broken diode in the rectifier, will give you a warning.

Benjo said...

"When off is selected the contactors switch in a dump resisitor and connect both sides of the magnet to the 0VDC rail."

I don't see it happening. I don't see the coil going directly to a zero-volt rail. At least, not in your schematic. By the way, is ve- supposed to be zero? Doesn't matter.

What I do see is... when the Lift contacts are opened, and the Drop contact closes, the coil is still energized, at a level determined by R_Coil in parallel with R3, and R1.

The magnet is essentially always charged as long as the system is on.

If the system is still on when you disconnect the plug, you WILL get a spark!

The schematic does NOT show both sides of the mag-coil going to ve- (assuming ve- is really supposed to be zero-volts).

The way I see it:
Lift - L on - full supply to coil
Drop - D on - low reverse power via current limit resistor R1 to help demagnetize the core.
Off - L and D off - Coil across R3 bleeder. One side still connected to -ve. If transformer has a static shield, it should be safe.

Benjo,

I don't see a "0 volt rail" in your diagram. The only time you willl have 0 volts anywhere is when both Load and Drop contactors are deenergized (Off), and enough time (decay time for inductor) has passed to bleed off the stored charge in the magnet coil through the resistor R3. Then the wires M1 and M2 should be at 0 volts, but who would want to touch it to find out? You need to insulate all bare spots, my friend.

Edit: Reading between the lines here, I think now that you are not talking about the magnet, but the power receptacle going to the magnet, which will still be hot and powered by the rectifier when and even if uplugged, and you are saying that the "0 rail" (-Ve?) of this 220 vdc power receptacle is exposed? If so, why not connect the "0 rail" wire to an earth ground somewhere?

If the supply transformer is ungrounded and has a static shield, then the entire supply is floating. No shock hazard.
If you ground one side of the supply, the other side will become a shock hazard.
I suspect the system is safe the way it is. I wouldn't want to start modifying until that was proven. The schematic above doesn't give any clues to how the supply is wired.
What he's calling zero volts is -ve. It's marked '-ve', not '0v'. That's my only clue that the supply is floating, and intentional for safety.

Keith, but what if it is ONLY the -Ve that is exposed? (There are some models of AC plugs-and-receptacles that have a "grounding pin" that is exposed to human contact, simply because there is no need to insulate a ground. What if they are using one of these for this application? Then would not it be practical to ground only that side?

Grounding -ve would make +ve 220 to earth. Right now, both are zero volts to ground.
Think of the supply as floating on the DC output side (+ve and -ve). The output of the transformer is the same as an isolation transformer. A static shield is needed to stop capacitive coupling. The shield and primary side can have grounding, but the secondary must float.
If there isn't a ground anywhere including the connectors, then the only ground would be inadvertant human contact. If everything is floating, no shock.
The plugs are DC, not AC, so I don't expect grounded plugs.
The lack of any grounding is intentional!

Thanks for all the replies everyone.

Just to clear things up... by 0V I meant the -Ve. If we are applying +220VDC to +Ve then -Ve will be 0V with respect to that (I did not mean 0v as in earth connection)

The operation is as keithkyll suggested:

Lift - L on - full supply to coil
Drop - D on - low reverse power via current limit resistor R1 to help demagnetize the core.
Off - L and D off - Coil across R3 bleeder.

There are two sets of plugs, one for M1 and one for M2. These each consist of one big pin and receptical.

My problem was that these plugs are connected to the -ve rail when both D and L are off. This is the position that the system is in when things are unplugged. (Users are aware that things must be left long enough to allow for current to decay).

I hadn't thought about the isolation properties of the transformer, and how the absence of an earth would prevent shock (as long as isolation remained intact). This has reassured me that this should be safe.

However there is still the worry that some one could unplug the magnet while it is on (when only L is activated). I can not however see any way to solve this.