Problem: Short Circuit Powers off the whole segment

[M. Atteya]

Hello Everybody,

We have a problem in our plant that we couldn't solve, we found that if the positive terminal of any spur touched the ground (Transmitter Casing, shield, .. etc) the segment power goes off and the whole segment is down !!. A reset is then needed on the power conditioner to re-establish the power again after solving the short issue.

We are using MOORE-HAWKE Device coupler "TG200" and TURCK power conditioner "RPC49-205".

Can anyone helps in solving this problem?

Thank you

[Michael Zeipert]

Hello M.Atteya,

The RPC49-205 is a not-galvanically-isolated Power Conditioner. Using a grounded bulk power supply and a device coupler with current limitation at the negative terminal could cause this problem.

Kind Regards
Michael Zeipert

[tklatt]

Hello M. Atteya,
an other possibility is the use of a galvanic isolated power conditioner.
In the solution recommended by Michael Zeipert it is not allowed to ground the minus lead of the bulk power supply (and if this lead is grounded depends on your cabinet layout).

Best regards

[Mike ONeill]

This is an issue that plagues anyone using a grounded power supply, or an isolated power supply that someone insists on grounding, with power conditioners that are not isolated. If you have such system and you deliberately ground one side in your control room, I don't think it is much of a surprise that an accidental ground of the other leg anywhere in the system causes a major short-circuit and the segment stops.

Isolated power conditioners might be more expensive and run hotter than the very simplest grounded system, but at least they survive accidental field ground connections.
This argument has been comprehensively discussed ever since people had the choice between Zener barriers and galvanic isolators; one is cheap and effective but can be blown accidentally, etc.

[hassan.elsayed]

Hello M.Atteya,

I have seen this problem before with a grounded bulk power supply connected to non isolated fieldbus power supply with one of the DCs system. What happens is that when a short circuit between the positive and the ground on the field device side occured, irrespective of any device coupler make, the return current bypasses the negative wire which is supposed to be going via the negative input of the device coupler. The device coupler will limit the inrush current and protect the segment from shutting down. Instead, the in rush current flows via the ground (cable shield) and has also bypassed the fieldbus power supply protection circuit which is also monitoring the return current. Some of the non-isolated power supply may loose the built in fuse, in your case, the power supply has been switched off. To solve the problem you need to isolate the bulk power supply before applying power to fieldbus power supply, or you need to use an isolated fieldbus power so that its output becomes floating. In either case, you can leave the ground connected to the negative bulk power supply, as far you have an isolate between the two.
Kind regards,
Dr Hassan El-Sayed

[M. Atteya]

Thank you for the replies

I've understood that the only way to avoid this problem is to use isolated power conditioners.

1- Can Michael Zeipert tell me about the best replacement for the RPC49-205 from TURCK to have an isolated power conditioner with the same features, i.e. termination, redundant power supplies, relay monitor, ... etc.?
or does anyone knows a good replacement from other manufacturers?

2-Does anyone know what will happen if we ungrounded the -ve leg of the power supply?

Thank you

[Michael Zeipert]

Dear M.Atteya,

you have a private message.

Kind Regards
Michael Zeipert

[hassan.elsayed]

Mr Atteya,


To answer Q2, let us first look at the definition of the "Ground" as stated in the NEC. " A conducting connection, whether intentional or accidental between an electrical circuit or equipment and the earth, or to some conducting body that serves in place of the earth." When talking about grounding it is actually two different subjects, earth grounding and equipment grounding. Earth grounding is an intentional connection from a circuit conductor usually the neutral to a ground electrode placed in the earth. Equipment grounding is to ensure that operating equipment within a structure is properly grounded. These two grounding systems are required to be kept separate except for a connection between the two systems to prevent differences in potential from a possible flashover from a lightning strike. The purpose of a ground besides the protection of people plants and equipment is to provide a safe path for the dissipation of Fault Currents, Lightning Strikes, Static Discharges, EMI and RFI signals and Interference.
So in your application, all segments are shielded, connected to ground. And presumably, the bulk power supply enclosure is also grounded, and your cabinet is also grounded. The idea to have the return of the bulk p.supply grounded is to have a reference point (low potential) specially if your input was based on AC/DC bulk power supply, You need some kind of protection/barrier to insure that no high voltage is likely be seen on the DC line. Then I suppose you can disconnect the negative from the ground, the system will still perform as normal.
Best regards,
H. Elsayed