Fusing vs Overload

[mrtweaver]

I am not sure if this is a topic for this group or not but I thought I would post it here and see what the general consenus is.

An older electrician here on site is preparing to retire. Two months and he will be gone. He and I are having a debate on fuses and overloads as protection.

We have 7 motors which all feed a trash collection system. We are interlocking them to the main drive motor on the system which is controlled via a freq drive.

If the main drive drops out it drops a relay out which supplies the 120Vac to contactors which will be connected between the mains and the 7 motors listed above.

Now if you look at the newer equipment we have purchased and even if you look at the older conveyor system. All of those engineers put in Fuses, then Circuit breaker, then contactor, then overloads then out to motor. To me this meets code.

The older electrician says the overloads are not needed that fuses will provide ample protection for the 7 motors. I say we need to put in overloads for added safety and code.

So the main question is who is right here, I say I am right and that code says you should put overloads on inductive loads such as motors and it should be done after the contactor. He says he is right and fuses will provide enough protection.

Thanks for reading and have a great day.

 

[briancr]

I am not sure if this is a topic for this group or not but I thought I would post it here and see what the general consenus is.

An older electrician here on site is preparing to retire. Two months and he will be gone. He and I are having a debate on fuses and overloads as protection.

We have 7 motors which all feed a trash collection system. We are interlocking them to the main drive motor on the system which is controlled via a freq drive.

If the main drive drops out it drops a relay out which supplies the 120Vac to contactors which will be connected between the mains and the 7 motors listed above.

Now if you look at the newer equipment we have purchased and even if you look at the older conveyor system. All of those engineers put in Fuses, then Circuit breaker, then contactor, then overloads then out to motor. To me this meets code.

The older electrician says the overloads are not needed that fuses will provide ample protection for the 7 motors. I say we need to put in overloads for added safety and code.

So the main question is who is right here, I say I am right and that code says you should put overloads on inductive loads such as motors and it should be done after the contactor. He says he is right and fuses will provide enough protection.

Thanks for reading and have a great day.

It seems you have answered your own question, I dont know the American wiring codes, but you have to follow them. Here in South Africa, we dont have to use fuses, but the rest have to be there

 

[TurpoUrpo]

1. fuse and overload are different things.
2. fuse and overload have different function.
3. as they have different function, they behave differently.

Overload protects the motor, fuse protects wires.

In here you need to put either overload or vfd per motor.

 

[jude]

In Canada most electricians use overloads to protect the motor and fuses to protect the wiring although the code does allow either fuses or overloads for overload protection of running motors, the old copy I have it is in table D16 and rules 28-300,

 

[luccass]

1. fuse and overload are different things.
2. fuse and overload have different function.
3. as they have different function, they behave differently.

Overload protects the motor, fuse protects wires.

In here you need to put either overload or vfd per motor.

Agree

 

[Christoff84]

It depends on what role each component was meant to play and is rated to play. I would guess the fuses are rated for the ampacity of the wire and act as short circuit protection. The overload would be rated for the motor and protects the motor from burning up the windings. I've rarely seen both fuses and circuit breakers in series with eachother on a single circuit as they both usually serve to protect the wiring, that being said, I have seen people put in breakers as a means of isolating the individual motors inside the panel. Without a wiring diagram to see how they setup the circuit, it would be hard to tell what is what.

For example, you could have 200A fuses, which go to a few 15A breakers, down to 7 motors each with an overload set to 1.9A

 

[JesperMP]

Provide a detailed example of such a motor starter circuit.
There is no splitting out to several branches after the fuses ? Or maybe a busbar after the fuses ?
Is there a change in wire or cable dimension from before and after the MCCB ?

In any way, sufficient short circuit breaking capacity must be achieved, but there must also be selectivity. You cannot have two devices tripping for the same fault.
If the MCCB has both shot circuit and overload trips, then there must still be selectivity with the overload after the MCCB.

Apart from that, this is how I see the differences between fuses, MCCBs, and overload relays:

Fuses = short circuit and overload protection. Today fuses are still used where the current levels are relatively high. This because fuses have superior current breaking capabilities at higher rated currents.

MCCBs = short circuit and/or overload protection, depending on what tripping device is built in. Used for the convenience of quickly resetting after a fault.

Contactor + thermal overload relay = overload protection.

I doubt that there is a code saying you must use fuses plus MCCBs plus thermal overloads. But you must somehow achive short circuit protection, overload protection, short circuit breaking capacity, and selectivity.

 

[gas]

Guys,
As stated above fuses and overloads are 2 very different devices and there is a big reason both are used.
Overloads should be set to trip at motor full load current after a time delay. The time delay is necessary because motors draw huge currents at start up. This protects the motor from overheating and burning up. It does NOT protect the process or people ONLY the motor.
Fuses protect wires and devices that should be heavy enough to provide for the above stated high starting current

 

[TConnolly]

Point out to your electrician (I'm using that term loosely here) that if only one fuse blows then the motor will single phase. Without an overload to open the contactor that motor may continue to run and eventually burn up. Fuses protect the circuit if a short circuit condition happens. Overloads provide thermal protection to the motor winding during a condition that creates heating of the motor coils (overload or single phasing, etc).

 

[Ned_Flanders]

Point out to your electrician (I'm using that term loosely here) that if only one fuse blows then the motor will single phase. Without an overload to open the contactor that motor may continue to run and eventually burn up. Fuses protect the circuit if a short circuit condition happens. Overloads provide thermal protection to the motor winding during a condition that creates heating of the motor coils (overload or single phasing, etc).

+1 On this.

Fuses (or circuit breakers) provide short circuit protection and protect the fixed wiring.
Thermal (Or magnetic) overloads provide overload protection for the motor.

It sounds to me like you should revisit previous work your "Electrician" has done - just in case.

I can't comment about your local electrical codes but British standards (17th edition wiring regulations) stipulate the maximum permissable motor size that can be used without overload protection. This is 0.37 kW.

 

[Binaural]

I have seen combination circuit breaker/thermal overload units. Would these meet code in the US/Canada? Example of the sort of thing I'm thinking of below:

http://www.schneider-electric.com.a...function_id=27&p_family_id=283&p_range_id=682

 

[Christoff84]

In Canada it would have to be CSA listed for that purpose, but as far as I can tell from the documentation it looks like it would meet the requirements for supplementary short circuit protection and overload protection.

 

[tracer]

aLARIC & Markus: I agree, fuzes with out an OL will single phase the motor, and then the odds are high that you will be changing out the motor starter as well as the motor after it burns up.....lol
From a PLC veiwpoint, I alwayz include a circuit from the motor starter to the plc to let the plc know that the motor relay is engaged. or to allow a fault to be displayed.