220VDC QUESTIONS

[Chas]

The project I am on has a 220VDC system with a floating ground.

1. What is a floating ground and why use it?

2. If I buy a DC panelboard, does it need to have 2-pole breakers to break the "+" and "-" lines?

Thanks for your help.

 

[don01]

Chas,
generally floating ground implies that there is no connection between the power supply and physical earth. It may also mean some **!!! has used a bloody autotransformer and elevated the 0 V rail above earth. so there is a potential to ground- generally it is the former.
In the funny world down under the short answer is YES.
I think it is advisable even if it aint mandatory in your region. If the 0v rail floats it MAY be above ground and lift the service tech above ground if he touches it.

What thoughts from the rest of the guys??
Don

 

[peterb]

Although you haven't said so, this sounds like a switchgear DC system (possibly an HV substation?).
Normal design of this type of system is for ungrounded (floating) operation; this has the advantage that a single ground fault doesn't trip anything (service continuity is of paramount importance for this duty).
Standard DC panelboard for this service would certainly have 2-pole breakers. Fused panelboards could also be used.

 

[jbartos]

Comment:
1. Floating ground is usually associated with ungrounded system. This system allows one fault to conductive (metal) surfaces without any danger. This must be taken theoretically only since in practice there are leakages and voltage levels must be considered, e.g. 220VDC is dangerous level, much higher than 24VDC considered relatively safe in dry locations. Electronics has additional reasons for floating grounds when there are differential voltages involved, e.g. differential amplifiers.
2. DC is consider much safer with 2 protective devices, one in + and other in -. In some instances, one protective device is allowed; especially if the - is chassis, e.g. car, but there is much lower voltage, 12VDC. Some relativity considerations could be considered for special cases where the power distribution would be inherently safe.

 

[Konstantinov]

Floating ground simply means that no point on the DC (or AC) system has been intentionally grounded.

Just recently, with the installation of our new switchgear, we converted from a 250 VDC ungrounded system to a 125 VDC ungrounded system. Our application is for operating circuit breakers on our 2,400 VAC switchgear. Over the years, it became obvious that 125 VDC was the more common voltage.

As was previously stated, an ungrounded system will tolerate a single ground fault, usually with no adverse reaction, unless there is a sensitive electronic device that is also referencing ground. Your question brings up a good point. It would be a good idea, and should be a simple matter to install ground detection/alarm on both the + and - bus. This would give the best of both worlds.

Since it is ungrounded, I would definitely install overcurrent protection in both the + and - bus. This way when the OC device is open, both sides are de-energized. Circuit breakers and/or fuses are the engineers choice, but I prefer fuses. With the lower current ranges, I can better coordinate both overcurrent and short circuit protection. Regardless of your choice, contact your preferred equipment vendor to confirm the suitability of and special ratings for panels, breakers and fuses when used on DC applications.