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16 comments
Associate

Join Date: May 2010
Posts: 46
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MPCB and Fuses Connected in Parallel

03/10/2013 12:47 AM

Recently I came across an old electrical drawing.It is a simple 3 phase motor feeder circuit consisting of a switch fuse unit, a power contactor and a thermal overload relay all in series with the motor as usual. The unusual part is there is a 3 ph MPCB / MCCB connected across the fuses. Can any body throw some light as to why such arrangement is required?

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Pathfinder Tags: parallel connection of fuses and MPCB
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Guru
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#1

Re: MPCB and Fuses Connected in Parallel

03/10/2013 1:07 PM

If the load sides are not tied together, maybe they intended to use it at a later date, or had money in a budget to use up so they needlessly spent on equipment. But if the load sides are connected and going to the exact same load, then it's a danger.

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Associate

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#2
In reply to #1

Re: MPCB and Fuses Connected in Parallel

03/10/2013 1:37 PM

The load is very much connected. If at all they wanted to provide the redundancy of protection, the MPCB should have been in series. If MPCB was shown as an alternative to fuses, it should have been shown as a dotted item. It is dangerous to provide this arrangement in the manner shown since neither the fuses nor the MPCB will serve their purpose and there will be no short circuit protection. ( The overload relay may provide some protection.)

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Power-User

Join Date: Jun 2009
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#3
In reply to #2

Re: MPCB and Fuses Connected in Parallel

03/10/2013 11:28 PM

- much depends on who has prepared the drawing and the legends for symbols used

- what may appear as a parallel MCCB on drawing may actually be a fuse monitoring device wired to give some control circuit command or condition indication

- if the drawing is an "as built" or "as commissioned" drawing then a good course should be to examine the equipment and compare it with the drawing - also look for the component list / bill of material in the drawing set - and the logic diagram too

- source of diagram shown - SIEMENS data sheet showing MPCB used as fuse monitor

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Guru

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#4
In reply to #3

Re: MPCB and Fuses Connected in Parallel

03/11/2013 12:51 AM

If two PDs are in parallel how will the load current get divided if both are on?.

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Guru
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#13
In reply to #4

Re: MPCB and Fuses Connected in Parallel

03/11/2013 10:37 AM

It seems the starting premise for the question is false.

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#10
In reply to #3

Re: MPCB and Fuses Connected in Parallel

03/11/2013 10:16 AM

Thank all. The 'fuse monitoring device' appears to be the right explanation. But then it may not be a MPCB, because it must have a very high resistance compared to the fuses so that when the fuses are healthy, a very small current will flow through the device. Only when one or more fuses blow, higher voltage will appear across the device and it can be used to trip off the motor power as well as control circuits. It can prevent single phasing in the event of a fuse blowing. Any idea what is the branch resistance of a MPCB? It should be higher if it has a thermal overload relay built in.

In this case I wonder whether we have different symbols for MCB, MCCB, MPCB and the fuse monitoring device so that the component can be identified easily.If some body gives the symbols for these four devices, it would help the designers to show correctly what is actually used. I sincerely thank all the contributors.

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Guru
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#11
In reply to #10

Re: MPCB and Fuses Connected in Parallel

03/11/2013 10:35 AM

<...Only when one or more fuses blow, higher voltage will appear across the device and it can be used to trip off the motor power....>

Actually, the fuses will do that all on their own. Such clever little things...

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#14
In reply to #11

Re: MPCB and Fuses Connected in Parallel

03/11/2013 4:14 PM
  • Actually, the fuses will do that all on their own. Such clever little things...

Actually to your actually, only one fuse clearing will NOT stop a 3 phase motor, it will single phase it.

There are better ways to protect against this situation than the silliness of this circuit, but maybe in days-gone-by they had to come up with this. I know that years ago, MPCBs did not have the same level of Short Circuit withstand capability that they now have, so it could be that they had to use fuses because of high available fault current, but did not want to risk single phasing the motor. Otherwise, why not just use the MPCB as the protective device in the first place?

A decent Solid State Over Load Relay will now guard against a single phase condition and SSOLs are barely more expensive than Bi-metal Over Load relays. There is no longer a need for a messy and complicated system such as what is shown.

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Guru

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#15
In reply to #10

Re: MPCB and Fuses Connected in Parallel

03/11/2013 10:53 PM

To reduce the current flow through the fuse monitoring device should there be a series resistance included in it?. Can you give a brand name?.

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Guru
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#12
In reply to #2

Re: MPCB and Fuses Connected in Parallel

03/11/2013 10:36 AM

Then it's something else and not a MPCB, whatever that means...

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#16
In reply to #12

Re: MPCB and Fuses Connected in Parallel

03/11/2013 11:28 PM

the product i had referred to in my post earlier was SIEMENS "Fuse Monitoring MPCB - 3VU1340-1MS00" . . . in the 1970s i recall using SIEMENS motor circuit breakers with fuse monitoring for the same application . . .

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Associate

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#5

Re: MPCB and Fuses Connected in Parallel

03/11/2013 1:03 AM

Dear what is the Motor rating??? Feeder DOL/ Star-Delta???. Fuse & MPCB rating selection are as per Type 2 or not???

It may happen that due to process design or Motor selection problem Fuses may gets blown at the time of starting so as to divert the current at start of the motor these shortcuts may have adapted. but again in such case your OLR should have trip first.

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Power-User

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#6

Re: MPCB and Fuses Connected in Parallel

03/11/2013 2:49 AM

The MPCB can give additional protection to the motors(which may be important for the motors) rather than thermal overload relay and fuses for example dry run protection,etc.

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Guru

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#7

Re: MPCB and Fuses Connected in Parallel

03/11/2013 5:11 AM

A good rule of thumb is that the thermal overload relay protects the motor while the fuses or MCB protect the upstream circuit from faults local to or in the motor. The situation described renders both the fuses and MCB useless as protection devices. It is dangerous and the control panel or the local isolator adjacent to the motor should be padlocked in the off position until the wiring is corrected.

It is surprising how threatened loss of production translates into a call for immediate action by senior management.

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Power-User

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#8

Re: MPCB and Fuses Connected in Parallel

03/11/2013 6:11 AM

To me it appears to be a 'fuse monitor'. Normally, when the fuse is intact very small current will flow through the fuse monitor and it will not operate. In case of blowing of fuse due to over current in any of the phases, current through the monitoring circuit which is connected across the blown fuse will increase and this will trip the fuse monitor. The auxiliary NO contact of the fuse monitor will be used to de-energise the contactor coil. Thus, the fuse will protect the motor against over current at the same time preventing any damage to the motor due to single phasing.

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Guru

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#9

Re: MPCB and Fuses Connected in Parallel

03/11/2013 7:03 AM

Yes it is fuse monitoring device. For MCCs supplied in 1980-1990we use to to incorporate for fuse monitoring.those days Siemens were making such device and it is not MCCB

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abele (1); debata07 (1); jhhassociates (1); JRaef (2); Muthuvel (1); pnaban (2); Priyadarshan (2); PWSlack (3); ramvinod (1); soebfatehi (2)

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