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from the Automation List department...
relay inquiry....
Engineering and workplace issues. topic
Posted by Andre Pablo G. Fausto on 15 June, 2001 - 1:36 pm
HI,

Just wanted to clarify. If you have a general purpose relay and the contact rating is 150V 10A DC and I would need to use it in an environment
that requires 220V 500mA DC, can I use this relay even if the voltage rating is incorrect?

Regards,

Andre Pablo "Apaul" G. Fausto
Nayon Kontrol Systems
354 (44) Quezon Avenue
Quezon City, Philippines
Tel. +63 2 4485074
Fax. +63 2 4485077


Posted by T. Connolly on 15 June, 2001 - 2:56 pm
No.
A relay voltage rating is independant of the current rating. The voltage rating applies to the insulation of the relay, the physical housing as well as the relay internal wiring and the coil insulation, which is usually a thin varnish applied to the coil wiring. It also applies to the contact ratings and ability to withstand arcing as the relay opens and closes. Exceeding the voltage rating can damage the insulation, and while your load may only require 500mA, if you damage the insulation, considerably more current will flow until either your fuse blows or you burn something up. Too much voltage at the contacts can damage the contact surfaces, independant of the current load, especially on DC applications. Once the contact surfaces begin to pit or burn the relay begins an ever increasing cycle of destruction with each operation. So unless you like rebuilding burned up controls, I wouldn't recommend it.


Posted by W. Mundy on 15 June, 2001 - 4:13 pm
Most relays have contact ratings that allows the voltage value to increase as the current value decreases. Potter & Brumfield (KUP model) relays are fairly standard, and they have a contact rating of 10 amps @120VAC and 6.66 amps @ 240VAC. Also, there are minimum and maximum current values on some relay contacts.


Posted by David P McConnell on 15 June, 2001 - 5:19 pm
The real problem with relays used to switch DC currents is their ability to interrupt the flowing current when the contacts open. Unlike AC (which goes to zero volts every half cycle) there will be a sustained plasma arc which can carry a lot of current as well as make a lot of heat.

The "interrupting" voltage rating on the relay contacts is the most important factor. I would be willing to bet that contacts rated to
interrupt 10A at 150 VDC would not interrupt 250 volt even at the reduced current. In fact, I would bet that it would arc and continue to carry the 500 ma current even past the point where most of the contacts had burned off the relay!

My two cents worth.

David McConnell
Stennis Space Center, MS


1 out of 1 members thought this post was helpful...
Posted by Phil Corso on 18 June, 2001 - 1:55 pm
Responding to David McConnell's Jun 15, 5:14pm reply on the subject:

David makes a good point about DC load current interrupting capability.
Thus, it is best to check with the relay manufacture because contact life (number of operations) is based on the time constant (L/R) of the load circuit being interrupted.

If you're unable to get the information from the relay manufacturer and the number and frequency of operations is low, i.e., in the order of 100
to 1,000 operations per year, then consider the following "Rule of Thumb." Series connecting two contacts will double the rated voltage
interrupting value. That is, connecting 2x150 Volt contacts in series will most likely work well in the 220 Volt circuit. From a practical
view point, however, one contact will suffer more damage than the other.

CAUTION:
There is one very serious concern. The insulation of electrical parts, and mechanical compnent spacing of the relay, example, insulated coil
leads, terminal to frame, contact to frame, etc, must be able to withstand the higher voltage.

For additional info on the subject of AC/DC contact rating/performance see previous posts on the subject.

Regards,
Phil Corso, PE
(Boca Raton, FL)


Posted by Rajesh Mehta on 18 June, 2001 - 2:43 am
Hi Andre,

As it appears from your question, you are talking about the CONTACT RATING and not the relay coil.

Well, first of all if somebody has a relay with 150V dc / 10A, I would like to have a look at that as such high current capacity I have not heard of unless we are talking of big industrial contactors (air circuit breakers). We used to connect the contacts in series (for arc quenching) to achieve such high current capacities.

OK, I would take that it is actually 1.0 A and not 10 A. If it is actually 10 A please make sure to check that there is no "purely resistive load" written in fine print. Because the characteristics may vary upside down in case of an inductive load.

Technically, there is nothing wrong in using it as 220V / 500 mA dc if the contacts were 150V dc / 1.0 A. Be sure to check the induction to resistance ratio or better known as L/R ratio to make sure it is less than recommended by the manufacturer.

Regards
Rajesh


Posted by Johan Bengtsson on 20 June, 2001 - 12:54 pm
Art. no 37-062-15 at elfa (http://www.elfa.se)
form 1C
16A, 250VAC, 4000VA
20A, 20VDC
0.3A, 300VDC
size: 29.4x12.5x25.2mm


Art. no 37-090-37
form 1C
10A, 50VAC
10A, 24VDC
min load current: 0.1A
size: 22.6x14.7x16.8mm


Well, that is just two of them....

/Johan Bengtsson

----------------------------------------
P&L, Innovation in training
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: johan.bengtsson@pol.se
Internet: http://www.pol.se/
----------------------------------------


Posted by Rajesh Mehta on 22 June, 2001 - 3:08 am
Hi Johan!

Maybe I missed but we are looking at 10 A at 150 V dc. If you notice, the one with 20A @ 20V dc has gone down to 0.3A @ 300V dc. You could put 150V dc somewhere in between. So my guesstimate is 1.0 A would be the upper limit @ 150V dc for this relay.

Regards
Rajesh


Posted by Gerald Moore on 18 June, 2001 - 10:30 am
In my opinion, it would be best to buy a relay with the correctly rated contacts.
You can, in a pinch, wire two contacts in series to increase the voltage rating. Since the voltage rating of the contacts is mainly based on the air gap between contacts, If you wire two contacts in series, the air gap is doubled. This assumes the rest of the relay is insulated for the voltage desired and that you have a spare contact. But, once again, it is best to buy a relay with the correct rating. Good Luck!

Gerry


Posted by McConnell, David P on 20 June, 2001 - 1:13 pm
The real problem with relays used to switch DC currents is their ability to interrupt the flowing current when the contacts open. Unlike AC (which goes to zero volts every half cycle) there will be a sustained plasma arc which
can carry a lot of current as well as make a lot of heat.

The "interrupting" voltage rating on the relay contacts is the most important factor. I would be willing to bet that contacts rated to
interrupt 10A at 150 VDC would not interrupt 250 volt even at the reduced current. In fact, I would bet that it would arc and continue to carry the 500 ma current even past the point where most of the contacts had burned off the relay!

My two cents worth.

David McConnell
Stennis Space Center, MS


Posted by Johan Bengtsson on 26 June, 2001 - 10:49 am
Sorry, my fault - yes I missed that somewhere...

They fall quite fast in allowed current when the voltage rises...


/Johan Bengtsson

----------------------------------------
P&L, Innovation in training
Box 252, S-281 23 H=E4ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: johan.bengtsson@pol.se
Internet: http://www.pol.se/
----------------------------------------

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