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Why Fuse Both Legs in a DC Control Cabinet?
Why would you want to fuse both the positive and negative side of a 24VDC control circuit?

Why would you want to fuse both the positive and negative side of a 24VDC control circuit?

We have a vendor supplying a control cabinet with a Siemens PLC and some I/O. Power to the cabinet is FLOATING 24VDC. The power goes to several circuits, each with a 5 amp fuse on the supply (+)side and a 5 amp fuse on the return (-) side.

When I asked why they had fuses on the negative side, they said it was because the power supply was floating. Can anyone help me understand why this is a good idea? I can't for the life of me draw a potential fault (or two) that isn't already protected by the positive side fuse.

0 out of 2 members thought this post was helpful...

grounds can occur on both sides of the contact. floating power supplies allow one to troubleshoot from merely a voltage level.

Once you figure this out, you will be grateful

By W.L. Mostia on 20 August, 2015 - 2:14 am
2 out of 2 members thought this post was helpful...

The below post does not help anyone on this mail list. One should not post unless you wish to help, inform, and/or educate the people on this mail list in a polite manner. That is the purpose of this mail list. There may be other people who would benefit from a good solid technical answer for this question. How about giving us one.


"> grounds can occur on both sides of the contact. floating power supplies allow one to troubleshoot from merely a
voltage level.

Once you figure this out, you will be grateful"


William (Bill) L. Mostia, Jr. PE

"No trees were killed to
send this message, but a large number of electrons were terribly
inconvenienced." Neil deGrasse Tyson

Any information is provided on a Caveat Emptor basis.

0 out of 1 members thought this post was helpful...

>The below post does not help anyone on this mail list. One
>should not post unless you wish to help, inform, and/or
>educate the people on this mail list in a polite manner.
>That is the purpose of this mail list. There may be other
>people who would benefit from a good solid technical answer
>for this question. How about giving us one.

>> grounds can occur on both sides of the contact. floating
>> power supplies allow one to troubleshoot from merely a voltage level.

Just recently I had a PE ask me why I checked balance on a 125 VDC circuit. I did not have the time to explain, I merely continued my work to ensure he could continue "on budget and on time".

Thank you "CuriousOne" but I do not see how that applies to this situation. A single ground fault on either side of the circuit in a floating system would not cause either fuse to fail. It would simply drive that leg of the circuit to 0V with reference to ground and the other leg 24V above or below it.

A second ground fault on the opposite leg of the circuit would cause a short that would blow the (+) fuse, and /or the (-) fuse if installed. Why would you need both?

Right now I only see it as an additional point of failure or leading to potential confusion during troubleshooting.

Thank You,
Frank

1 out of 2 members thought this post was helpful...

Frank,

From your other thread, if the 24 VDC is truly floating then it's reasonable to fuse both phases (legs) of the supply. If the negative leg were grounded, one would not need to fuse the negative leg, only the positive leg.

It would seem the supplier is trying to protect the floating 24 VDC supply from overcurrents in various circuits, not necessarily grounds. A solid ground on either leg isn't going to blow the fuse on a floating DC supply. Only if both legs were grounded would excess current flow through the fuses to cause the fuses to open (blow).

Don't know if this answers your question--but to my mind the only reason not to fuse the negative side of a DC supply would be if the negative side were grounded. And that would make the DC supply NOT floating.

Floating supplies are used to be able to allow a ground on either leg (positive or negative) to exist without causing a problem. Fusing is not generally done to protect against grounds, but to protect against over-current--which may be the result of a ground (or two in a floating system), but not necessarily.

>From your other thread, if the 24 VDC is truly floating then
>it's reasonable to fuse both phases (legs) of the supply.

>
>It would seem the supplier is trying to protect the floating
>24 VDC supply from overcurrents in various circuits, not
>necessarily grounds.

CSA,
I still am having trouble seeing this. Can you give me an example of a floated DC circuit where an over current in the load for whatever reason would blow the negative fuse and not the positive one? I keep seeing current being a loop that has to come from the positive terminal and end up back at the negative terminal so any over current will have to come from the (+) side of the battery and go through that fuse, no?


Thanks,
Frank

0 out of 1 members thought this post was helpful...

CSA is on track but not giving us his all.

Many 125 VDC bus on power plants are floating. A ground detection system is provided to reference half the voltage to ground.

When a ground occurs, we can actually detect if it is on the negative or positive portion of the bus. I have actually seen balancing grounds on a large bus. ground on + balance ground on - .

All floating power supplies can be set up the same as I described above.

2 out of 2 members thought this post was helpful...

Hi, Frank. Perhaps the following page from Schneider Electric Catalog will clarify:

https://drive.google.com/file/d/0B_tXNJBTu4ZPMzhNbVk4VVNFcUU/view?usp=sharing

As mentioned in previous link, type of DC network defines the number of poles a breaker must break when a certain fault occurs. In the case of Earthed network (network where one pole, usually Negative pole (-), is earthed), only L+ is to be broken in case of fault. It is not necessary to break negative pole because by short-circuiting the negative pole no fault current will occur.

However, in the case of Earthed central point (for example usually control circuit voltage in power plants in Croatia is 220 V DC, central point is earthed and one pole is at +110 V DC and the other at -110 V DC) and in the case of Isolated poles (floating DC source, as you mention), the fault current will occur by short-circuiting either the negative (-) or positive (+) pole to Earth. So it is important to break both poles in case of fault.

Bear in mind that when designing DC network, you should always use DC circuit breakers (sometimes, designers use AC due to being less expensive), and that you connect them properly regarding positive (+) and negative (-) terminals, especially 2-pole MCBs (always follow data given by manufacturer). It is crucial to determine the direction of energy flow (where is source and where is load). I have some experience where wrongly connected 2-pole DC MCB burned out during short-circuit test.

Best of luck!
I.H.

>
>Best of luck!
>I.H.

Thanks I.H.

I appreciate the grid style layout of the examples. The floated example shows "not relevant" for fault types A and C which make sense because either leg to ground would not cause a fault. Fault type B, leg to leg short, would cause an over current, but breaking a single leg would clear the fault.

The Schneider document got me thinking though, as it's for breakers and not fuses. Maybe it is more for maintenance or maintenance safety instead of strictly overcurrent protection.

With a floated system I can see that potential exists for a fault to occur on either side of the loop and no matter what side you put the fuse on, the other side could potentially provide a hazardous voltage at the point of the fault even after one of the fuses has blown, if there were un-intentional grounds. I can see why a two pole breaker would provide good protection here as it opens both sides of the float and wouldn't leave a potentially hazardous voltage on either side, but fuses don't offer the same safety. Once a single fuse blew, you would want to pull the other fuse for safety before you checked the circuit, but the second fuse wouldn't provide any additional over current protection that the first fuse hadn't.

By Curt Wuollet on 23 August, 2015 - 12:37 am

One poor practice begets another?.

Regards
cww

Does the machine have a mixture of npn and pnp devices? It is most common to ground the negative DC in a machine which uses pnp devices (sinking inputs, sourcing outputs), and I believe that machines with npn devices it would be common to ground the positive DC. If they have a mixture of both types of devices, this could be a reason to leave the DC circuit floating.

In case of a digital contact wiring, If the common looping is external to the panel terminal or output load common looping is followed external to the panel terminal, then a fuse at the return terminal will help if the optocoupler or a shorting at the module output which can deliver a high short circuit current at the return terminal in case of digital input or common supply point in case of digital output.
Such cases we can visualize a voltage floating situation at the terminal.

This is not because of an issue from the field. Instead a protection invade of malfunction in the module. Normally adequate protection should have been given in the module without a fuse at the return.
Also a detail vomment can't give without checking the loop drawing. But above explanation indicate the are reasons behind.

Simplified answer...

With a floating supply, as noted, first or subsequent ground fault(s) on one leg does nothing other than ground the supply. First fault to ground on the second leg, blows both the positive AND the negative fuse when both legs' fuses are a single and same rating fuse in the fault current path (assuming both are fast-blow fuses). This will indicate which circuit conductors (or equipment) on which the faults have occurred. If you have several ground faults on one leg, those circuits' fuses will parallel the fault current and may not blow. So if you only have one leg fuse blow, you should be looking for several ground faults on the other leg. When both positive and negative fuses blow, and clear the fault from the system, the system is returned to floating status... while the faulting circuit and or equipment can be repaired or replaced.

0 out of 1 members thought this post was helpful...

A: Rarely, if ever, have I seen more than one fuse blow in the same circuit.

B: Fuses are added to circuits as current limiting devices not troubleshooting tools.