Vacuum contactor without protection fuse 5

[Eng.Edward.M]

I have a doubt, I hope you can help me.
My client has a vacuum contactor with a fuse to protect a 1800kW motor, but this fuse was removed and copper bars were installed in its place.
This fuse protects against short circuit, correct? (function 50). If I install a relay with 50/51 functions, would that solve the problem? or should i install the fuse again?

 

[dpc]

The fuses are almost certainly required. The contactor cannot interrupt fault currents and the starter is specifically designed and rated based on the fuse clearing before the contactor drops out. If you add a relay, what is it going to trip? I suppose if there is a dedicated upstream circuit breaker, you might be able to get it to work, but I doubt it. At high values of fault current, the fuses will be much faster than a circuit breaker. This starter is a disaster waiting to happen, at least as you've described it.

 

[che12345]

Dear Mr. Eng.Edward.M (Electrical)(OP)17 Jan 23 18:26
"#1. ......My client has a vacuum contactor with a fuse to protect a 1800kW motor, but this fuse was removed and copper bars were installed in its place......This fuse protects against short circuit, correct? (function 50).....#2. If I install a relay with 50/51 functions, would that solve the problem? or should i install the fuse again? "
1. I am in full agreement with the learned advice by Mr. dpc . Remove the copper (bare?) bars and reinstate the fuses immediately ! You are having a "time bomb" . The bare copper bars would short across the phases, are going to be extremely explosive and fire in the enclosure; in case of a short-circuit at or any location including the cable and the motor.
2. No relay , even breaker at the up-stream would help with this illegal bare copper bars.
Che Kuan Yau (Singapore)

 

[Eng.Edward.M]

Thank you for your help.

I would like to understand, because the relay cannot do this protection? The contactor is from ABB model VSC7/P and with an ABB REM 610 relay, would this relay not send a signal to the contactor and with that the contactor would turn off? or because of the high short-circuit current this contactor would not support opening, is that it?

 

[LionelHutz]

The contactor probably can open something like 5kA. So, clearing any fault higher than that would cause the contactor to fail. Also, the contactor is not designed to hold in during a fault through a number of cycles. It's designed to have fuses clear the fault before the first 1/2 cycle of peak current reaches it.

 

[Parchie]

@Eng.Edward.M,
You may have bought the version of the ABB vacuum breaker with the option of not using fuses. The shorting bus bars are included in the packaging accessories. Methinks, there must be an upstream OCPD in place that protects the conductors supplying the VCB, the VCB itself, and the motor. I doubt such an expensive, big motor is not protected by a fuse or breaker upstream.
The only instant that we are not required to fuse is when the source is an inverter or converter. This is because module manufacturers are required to have an integrated OCPD in their modules that protect the module (and its load)by at least 156% of the short-circuit current that the module can provide.
Hope that helps.

 

[LionelHutz]

Contactor sheet I found said 6kA stand alone or 50kA with fuses.

 

[RRaghunath]

It boils down to - what is the fault level at the motor switchgear!
If the fault level happens to be under 6kA the contactor is capable of interrupting and doesn't need backup fuses.

 

[dpc]

I've never heard of the ABB configuration that Parchie refers to. But I remain a little skeptical. The contactors are designed to delay opening on a fault to allow the fuses time to clear. I'm not sure how that would work with a 3-cycle or 5-cycle circuit breaker. I've seen a lot of medium-voltage motor starters and I've never come across one with slugs instead of fuses, here in the US.

 

[RRaghunath]

The Vacuum contactors of Siemens that I have come across had 400ms delay in opening of the Contactor.
This is built in to the electronics that control the Contactor solenoid. But, Siemens also offer the option of disabling the delay at site.
When the earth fault current is limited by NER in MV system (say, medium resistance earthing), the fuses are not effective in protecting the feeder against earth faults and the delay in opening of Vacuum Contactor could mean increased damage at the point of fault.
In case of three phase fault that yields fault currents in excess of Contactor breaking capability, Fuses blow / open the circuit before damage can take place to the Vacuum Contactor contacts.

 

[che12345]

@ RRaghunath (Electrical)21 Jan 23 05:25
" #1. ... When the earth fault current is limited by NER in MV system (say, medium resistance earthing), the fuses are not effective in protecting the feeder against earth faults and the delay in opening of Vacuum Contactor could mean increased damage at the point of fault....}"
Agreed.
Any fault current that is within the switching capacity is detected by the relay and is switched off by the contactor. The fuse is not an over-load nor an (low EF with NGR) protection device. The main function of the fuse is to protect the contactor, which has a low SC kA rating.

".#2...In case of three phase fault that yields fault currents in excess of Contactor breaking capability, Fuses blow / open the circuit before damage can take place to the Vacuum Contactor contacts...".
Now the fuses are replaced by bare? copper bars.
This is the issue. What is your advice?
Che Kuan Yau (Singapore)