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Relay contact rating question

Relay contact rating question

Relay contact rating question

Would someone be able to tell me what this means on a specification for relay contacts: "......10 amp make, 1.5 amp break."
Is this the same thing as saying the contacts need to be rated for a 10 amp continuous load?

The rest of the relay spec seems clear, coils may be either 120VAC or 24VDC, din rail mount, etc. etc.

We're trying to meet a boiler plate specification written by consultants and the RFI process isn't always helpful. Usually when asking what do they mean by xxxxx, the response is "provide per the specification,." which really does not answer the question.

RE: Relay contact rating question

The difference between make and break current is usually valid when the current is DC. It is a lot tougher to break DC in an inductive circuit than it is to break AC.

Not possible to answer if you do not provide all the pertinent facts. Like AC or DC, voltage, load type and such things.

Gunnar Englund
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Relay contact rating question

Thanks for your answer Skogsgurra.

Below are the details you were asking about:
The coil for each of these relays will be 120VAC.
The loads that a relay contact will switch is going to vary from a 120VAC motor starter coil, 120VAC or 24VDC indicator lights, 24VDC relay in a VFD, etc. There may only be one device attached to each relay contact.

One relay will be connected to each point of an AB 1756-OA16I 120VAC isolated discrete output module per their specifications. And yes, I realize it seems to be a little redundant using isolated modules AND relays but this is for a WWTP and they are concerned about lightening strikes.

The specs also require each discrete input to be first wired to a 120VAC relay coil and then the contact from that relay go to a point on an AB 1756-IA16I isolated discrete input module.
So my question remains: With the specification language being "......10 amp make, 1.5 amp break" are they asking for the relay contacts to have a 10 amp continuous rating?

Thanks in advance for your help.

RE: Relay contact rating question

Not necessarily.

You did give the type of load that you actually have. But you still didn't say anything about the load that the relay specification is based upon.

Many loads have a substantial inrush current. Rectifiers with large capacitors, incandescent lamps and motors are among these loads. There, a 10 A making capacity doesn't always mean that the contact can carry 10 A continuously. You need to look for the complete specification. It should contain the following data:

Voltage type (AC or DC), working voltage (very often 250 V or 600 V if AC and usually a lower value if DC), making current (your 10 A), continuous current (e.g. 6 A or just about anything), breaking current (your 1.5 A, which probably is DC) and also the type of load that the relay can handle (resistive, inductive with time constant or cos(phi), capacitive etcetera. Sometimes, there are also a set of data for "dry circuit" applications and that is the lowest recommended voltage and current that you can reliably switch with the relay. If voltage and current too low, the contacts may not clean themselves properly.

Gunnar Englund
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Relay contact rating question

In the "good old days", we here in the USA used to use commonly understood NEMA classifications for things like this. So in the old "NEMA Pilot Duty" system, the Engineer would have likely called out for NEMA B300 or C150 ratings (assuming AC), something like that. That would have then immediately classified the Make, Break and Thermal design criteria as well as the voltage. The letter was the contact current rating, the number was the maximum RMS votlage.
A = 10A Thermal (aka continuous), 60A make, 6A Break;
B = 5A Thermal, 30A Make, 3A Break;
C = 2.5A thermal, 15A Make, 1.5A Break.
The 150 / 300 / 600 after the letter was the maximum RMS votlage.
So a NEMA C150 would be closest to what your spec asks for.

Now however, because NEMA ratings mean nothing outside of North America, manufacturers make whatever the heck they think they can get away with and no longer design TO a specification. They design first, usually to an ECONOMIC criteria, then test and label according to any industry ratings they happen to meet, or simply ignore them if they don't meet or exceed them. So for example I have looked at one mfr who shows the same style relay as having a C300 NEMA rating, but they claim that the Thermal is 7A in one version, but 10A in another version (different part numbers). So what that means is that they are FIRST claiming that the two versions are 7A and 10A, but when subjected to a standardized test criteria, such as the NEMA Pilot Duty ratings, then neither of them passes anything more than NEMA C300, which is only 2.5A Thermal, 15A Make, 1.5A Break at 300VAC RMS. Bottom line, the 7A or 10A thermal rating, to them, means nothing against a standardized design criteria, so you are on your own as to whether you believe it or not. It may be that they EXCEED the thermal design criteria for NEMA B300, but only by virtue of their own test criteria, which you will not know. But as far as it's switching capacity, it must not have exceeded the B300 design criteria, otherwise they would have said so.

So what your specifying engineer has done is to say "I don't really care what marketing terminology spin your supplier uses to describe the thermal rating of the contacts, because I know that is somewhat nebulous anyway. Instead, I insist that they at LEAST be capable of Make 10, Break 1.5.".

"Will work for (the memory of) salami"

RE: Relay contact rating question

Be aware of a potential pitfall. Most solid state AC output modules have enough leakage current to hold in AC relays! So you will have a situation of make once and forever hold. Much better to use solid state DC outputs and DC relays.

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