Overcurrent protection in DC circuits.

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Is it necessary to provide overcurrent protection on both sides of the load in an industrial 24VDC circuit? I've not found anything in the code suggesting you do, but many of the ex-mariners I'm working with say that this is the case on ships. I realize that marine electrical codes are their own entity, but I wanted to ask here about what the NEC might say.
 
Ultrahighgain said:
Is it necessary to provide overcurrent protection on both sides of the load in an industrial 24VDC circuit? I've not found anything in the code suggesting you do, but many of the ex-mariners I'm working with say that this is the case on ships. I realize that marine electrical codes are their own entity, but I wanted to ask here about what the NEC might say.

If one leg is grounded, then no OC/SC protection is required in the grounded leg.

Ships do not fall under the purview of NEC.
 
weressl said:
Ships do not fall under the purview of NEC.


I said that, but thanks.

Do you have some code references for me illustrating "no overcurrent protection in the grounded leg required?"
 
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Ultrahighgain said:
I said that, but thanks.

Do you have some code references for me illustrating "no overcurrent protection in the grounded leg required?"

There are numerous, depends on what they supposed to do.

230.90. (B) Not in Grounded Conductor.​
No overcurrent device
shall be inserted in a grounded service conductor except a
circuit breaker that simultaneously opens all conductors of
the circuit.

240.15 Ungrounded Conductors.
(A) Overcurrent Device Required.​
A fuse or an overcurrent
trip unit of a circuit breaker shall be connected in
series with each ungrounded conductor. A combination of a
current transformer and overcurrent relay shall be considered
equivalent to an overcurrent trip unit.

 
weressl said:
There are numerous, depends on what they supposed to do.

230.90. (B) Not in Grounded Conductor.​
No overcurrent device
shall be inserted in a grounded service conductor except a
circuit breaker that simultaneously opens all conductors of
the circuit.

240.15 Ungrounded Conductors.
(A) Overcurrent Device Required.​
A fuse or an overcurrent
trip unit of a circuit breaker shall be connected in
series with each ungrounded conductor. A combination of a
current transformer and overcurrent relay shall be considered
equivalent to an overcurrent trip unit.



Hey...thanks for your help.
 

templdl

Senior Member
Location
Wisconsin
When it comes to using MCCBs for a DC application other that the ungrounded or grounding issue being addressed 24-48vdc should be an easy application for MCCBS if they have a DC rating.

It is common for a single pole of a breaker to be rated up to 125vdc. Another reason to series the poles of a 2p or 3p breaker is to increase the dc voltage rating to 250vdc or possibly more. Because DC current doesn't go through 0 volts like AC current does it is more difficult to clear a DC arc. Putting the poles in series essentially increases the contact gap. The poles can be either looped together on the positive or negative side or one or two poles series on the plus side and the other on the on the negative which also provide isolation when the OCPD is opened.
 
Ultrahighgain said:
Ok...I was wondering if I could get away with fusing only the + side of the load(s) since this is considered low voltage.

Yes and No depending on what type of DC power supply and set up.

And if this is just a two wire DC or Three wire DC system it can get tricky with the OCPD set up.

Typically with most two wire DC system only one conductor need to be fused or CB'ed but however Three wire DC the bet is off it will varies widely depending on itself.

Merci,Marc
 

KentAT

Senior Member
Location
Northeastern PA
At my facility, the older 24VDC circuits have a grounded common. In these, the voltage is +24VDC from the positive conductor to the other conductor, which we call the common. Also reads +24VDC from the positive conductor to grounded conduit, JBs, etc. In these, I see the designers only have fuses on the + side of the circuits.

Most of our newer 24VDC systems are installed with floating grounds. The voltage from the positive conductor to the other conductor is +24VDC. The voltage from the positive conductor to plant ground is +12VDC. The voltage from the negative conductor to ground is - 12VDC. For these systems, both sides are fused.

Kent
 
KentAT said:
At my facility, the older 24VDC circuits have a grounded common. In these, the voltage is +24VDC from the positive conductor to the other conductor, which we call the common. Also reads +24VDC from the positive conductor to grounded conduit, JBs, etc. In these, I see the designers only have fuses on the + side of the circuits.

Most of our newer 24VDC systems are installed with floating grounds. The voltage from the positive conductor to the other conductor is +24VDC. The voltage from the positive conductor to plant ground is +12VDC. The voltage from the negative conductor to ground is - 12VDC. For these systems, both sides are fused.

Kent

Again, it would be a great convenience to not have to fuse the return i.e. the (-) or 0V side of the supply...but I can't find any specific code referance to permit it.
 

ELA

Senior Member
Occupation
Electrical Test Engineer
KentAT said:
Most of our newer 24VDC systems are installed with floating grounds. The voltage from the positive conductor to the other conductor is +24VDC. The voltage from the positive conductor to plant ground is +12VDC. The voltage from the negative conductor to ground is - 12VDC. For these systems, both sides are fused.

Kent
Would this be a floating ground or a center tapped ground?
Fusing both sides makes sense in a center tapped ground.

I have only experienced one fuse in the (+) lead of lower voltage DC supply lines that are not center tapped.
 
ELA said:
Would this be a floating ground or a center tapped ground?
Fusing both sides makes sense in a center tapped ground.

I have only experienced one fuse in the (+) lead of lower voltage DC supply lines that are not center tapped.


Ours is to be floating.
 

ELA

Senior Member
Occupation
Electrical Test Engineer
I am unsure of what standards might address this.

In a practical sense I would say that a fuse is only required in one of the two leads.

Since the system is not grounded, no appreciable current will flow when either lead is shorted to ground. The only potential difference is from the (-) to the (+) lead. As long as one of them is fused I would consider it protected.
 
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