Surge Suppressors on Industrial Type Relays 5

[nightfox1925]

I would like to have an idea on what is the purpose of surge protection on industrial type relays.

I am using an industrial type relay as an interposing relay for closing a circuit breaker. The contacts on this relay are well rated for the breaker closing coil duty at 125Vdc.

The coil is energized by a 120VAC UPS. Is surge protection required in this set up?

 

[Skogsgurra]

The purpose of surge supression is to avoid having the inductive energy doing damage when interrupted. There are three kinds of damage that occur: Arcing, which wears contacts . High voltage peaks, which may destroy semiconductors or cause flash-over. High frequency oscillations due to repeated discharge of the capacitance in the coil and wiring, which will upset fast electronic circuits.

An industrial relay or contactor will usually not cause any problem with arcing or high voltage peaks. But it will always cause HF interference, which may or may not upset electronics in the vicinity. I have had quite bad interference from a standard 11 pin relay that occasionally did reset a PLC working a couple of meters from that relay. An RC snubber or a varistor across the coil is an effective remedy.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[nightfox1925]

Thanks skogs for the reminder.

I have another concern. The spec of the relay installed at the VFD control cabinet with the PLC and LDA is as indicated in the link below:

http://www.ia.omron.com/data_pdf/data_sheet/mk-s_ds_csm1382.pdf

I am applying the N.O. contact to close a 7.2kV cirucit breaker with a coil rating of 3A at 125Vdc.

The specs says 10A at 250VAC resistive and 7A at 250VAC inductive, 10A at 30Vdc N.O. resistive and 5A at 30Vdc N.C. resistive.

Since I am applying it to a 3A 125Vdc breaker closing coil for a pulsed closing command, can I say that:

The relay breaking capacity at 30Vdc is 5 x 30 = 150VA

The relay breaking current would be 150VA/125Vdc = 1.2A

Is tje above calculation correct or the relay contact will work fine. If not, is there any other model or make that I can use for this application?

 

[nightfox1925]

We decided to replace the relay with KUEP-3A15-120 which has a DC current switching of 10A at 125Vdc.

However, I am looking for the same type with the varistor across its coil. I'll contact Potter Brumfield (Tycoeletronics) if they'll have one of the same type so I can simply replace the relay not the socket base.

I don't know if Omron has a 10A, 125Vdc duty industrial relay...does anyone have any idea?

 

[waross]

The varistor across the coil will protect the driving circuit. (PLC?) You may consider another varistor or freewheeling diode across the contacts to limit high voltage transients across the contacts and across the circuit breaker coil.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[slavag]

Hi nightfox1925.
Please check those types:

http://www.findernet.com/comuni/pdf/S65EN.pdf

http://www.schrack.com/tools-downloads/catalogues-brochures/schrack-relays/en/

Please take in account, it's making, breaking 10A 125V dc...not seems, you disconnect this by 52a contact.

Godd Luck.

Slava

 

[ScottyUK]

If you are looking for a breaker tripping relay then consider types like the Clifford & Snell D2900 primarily designed for interpose close and interpose trip applications. They are well made and reliable.


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If we learn from our mistakes I'm getting a great education!

 

[nightfox1925]

The potter & brumfield KUEP-3A15-120 relays were used by ABB in their 7.2kV switchgear circuit breaker controls for remote close and trip.

Having this in mind, we also used the same relays for the VFD remote close and trip (the 7.2kV switchgear feeders are supplying power to the 7.2kV VFDs).

The VFD has "pre-charging" requirement and the pre-charging is controlled internal to the VFD using 600VAC via pre-charging circuits. We decided to install the pre-charge control using interposing relays and is fed from the same 120VAC UPS supplies that is available inside the VFD control cabinet. The manufacturer cautioned us that if we use the 120VAC UPS supply feeding their LDAs and solid state controls, we have to

1) provide "surge suppression meanings" to the relays we are installing.

2) physically install the pre-charge interposing circuits separately within the control cabinet.

I think the reason why they want surge suppression meanings on the relays we are installing is to protect their sensitive electronic components against voltage spikes when the coils are open circuited during dropout operation.

Unfortunately, we ordered the relays without varistors or surge meanings on them. We have complied item 2) above. If we provide a separate external 120VAC for our "pre-charging circuits" instead of tapping to their 120VAC UPS power supply, can we be able to make it work without these surge suppression meanings?

 

[LionelHutz]

Just buy a ~170VDC MOV, probably about 20J or so would work.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=P7205-ND

If you really want to provide protection, then install a RC snubber. Some motor contactor manufacturers will also make a small cube RC snubber with a wire leads to add to their contactors.

I think you missed the detail from slavag - The relay only has to make the 125VDC. You describe breaker closing so there will be a 52B contact from the breaker in series with the closing coil. You energize the coil with your relay and then the 52B will break the current once the breaker closes. So, since DC is hard to break and not as bad to just make, the relay you first posted should be fine. You can put 2 or 3 contacts in series if you're really concerned.

 

[nightfox1925]

Thank you gentlemen for all the assistance. Now we are on the right direction on this.

 

[ozmosis]

As it is going in a VFD control system (I assume) then recommendations are always there to suppress the relay. I have seen so many failures and spurious problems with un-suppressed relays causing high noise in the whole control system. I've measured in the region of 6kV burst transients as a result of un-suppressed relays. This is a big problem to those sensitive VFD control inputs that would be rated at best to 4kV transients and typically for the general purpose 2kV.
For a few $'s, it is worth doing. Agree with LionelHutz about the RC snubber.

 

[nightfox1925]

Thanks Ozmosis. Let me ask a question then.

The only input to the VFD card that is part of the 120VAC pre-charging control that we are making is the pre-charge relay (PR).

This pre-charge relay (PR) is a 120VAC relay Omron #MKS. This relay is energized by a momentary closure from a 52CS. The relay is wired with a seal-in contact so it sustains itself once it is energized by that switch. Its dry N.O. contact closes and sends an initite command to the VFD digital input to start pre-charging process. After the VFD PLC indicate a pre-charging complete, the updtream circuit breaker is closed. A 52b contact is wired in series to the PR relay coil to drop it out after a successful breaker closure and takes out the pre-charging input command.

Since the PR relay 120VAC power supply is fed separately from an external AC supply and physically installed separately from the VFD electronic circuitry, is this still going to cause transients on the VFD electronic circuitry?

I have ordered the RC snubbers and we will installed them on these relay coils later.

 

[Skogsgurra]

The answer is Yes. The arcing contains MHz components that are coupled across isolating barriers. Physical and electrical separation is no guarantee that you can avoid interference.

OTOH, if the relay is part of the inverter, there should already be a snuber built into it.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[nightfox1925]

Wow that's a show stopper. We are pretty much squeezed big time in our time table and the pumps has to be bump tested at least today.

Os there anybody who can suggest an RC snubber that we readily purchase within Canada, specifically Fort MacMurray Alberta?

 

[nightfox1925]

Gunnar,

I have seen an RC snubber on one of the VFD panel 120VAC relays that was installed as part of vendor supplied equipments that is not yet scheduled for commissioning till next week.

I am planning to temporarily to take that RC snubber and install that temporarily across our 120VAC relay coil. Once we pre-charge and energize this VFD we are working on, we will re-install the RC snubber back. Looks like a contingency plan...but will it be compatible with our MKS relay?

 

[Skogsgurra]

Anything between .1 and .47 uF with a 10 - 100 ohm series resistor will work well. Data are not critical.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[nightfox1925]

Thanks Gunnar,

I finally got a quote for the RC snubbers: 0.1uF, 150 ohms, 125VAC from CDE part number #104MACQRL150.

Thanks for all the responses and advises.

 

[davva]

I wonder if I can tag on to the end of this topic (or should start new subject). Seems relevant to post here.

We have recently had some EMC measurements taken on a switchboard and found that a lot of common mode interference is being generated.(The interference lead to a black out hence the EMC survey to locate the problem). The interference seems to be concurrent with the operation of a particularly relay coil energising and deenergising (interference spikes occur at the same time as the audible noise of the relay operation is heard).

We have found that the relay is fitted with a varistor across the coil (ac 220V coil). My question is would an RC suppresion coil be better? The relay manufacturer allows for the fitting of a varistor or a RC suppression unit on the relay.

I have read that disadvantages of varistors are that they are not effective below Uvdr and also have limited lifespan. Disadvantages of RC suppression units are limited lifespan.

I would be interested to hear in any similar experiences or thoughts.

Is it also possible the varistor has failed in our instance? Due to limited access we have not been able to prove this yet.

 

[nightfox1925]

Are you referring to relays installed inside the switchboard and breakers or protection relays on that switchboard tripped for no apparent reason. If you say switchboard, are you refering to an eletrical switchboard?

OR are you refering to relays installed near your electronic equipment which also controls the same switchboard remotely?

 

[davva]

Switchboard is a 6.6kV switchboard. Relays are 220V coil relays with 2n/o and 2n/c auxiliary contacts which are fitted in the control section of the switchboard. Varistor is fitted across 220Vac coil on relay.

Black out was caused by interference effecting frequency transducer in the same switchboard. Frequency transducer has been replaced with a different type and problem (which was repeatable) has gone away. The electrical noise remains on the circuit though (i.e. the relay operation is still creating common mode noise)