need a two pole solid state relay for 220volt?

[petervb]

I'm designing a controller for a small process heater, switching 220volts at 20amps. Since the controller uses a ramp/hold pattern with a lot cycles, an SSR is recommended for reliability. Question is: should it have a two pole relay? the 220volt is the north american "two phase" system with 220v line to line and 110volt to ground. The risk is that with only one pole the heater has 110volts to ground at all times. Certainly mechanical contactor designs are two pole in my experience, say for HVAC compressors, but adding a pole to an relay is relatively cheap, not so for an SSR

 

[Compositepro]

220 in North America is single phase not two phase. A solid state relay will have some leakage so it is not useful for voltage isolation, i.e. a high impedance voltmeter will still measure 120 volts to ground after a SSR in the "off" state, unless there is low impedance load to ground. If you want you can just use two single pole SSR's, one on each side of the heater. SSR's usually fail in the on state so for safety a two pole electro-mechanical contactor should be in the heater circuit, which is controlled by a latching over-temperature controller. This a is a legal requirement for industrial ovens in the U.S. An SSR is preferred for the normal, quick cycling of the heater for temperature control.

 

[itsmoked]

220V left with the Dodo.
If the heater is 240V then there is no neutral involved. A single pole SSR is normal. But as Comp mentions you should include a mechanical relay controlled by something other than the main temp controller (preferably) that is indeed latching so it requires human intervention after tripping. It usually is associated with a START button that then latches the contactor in.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[petervb]

Thanks compositepro and itsmoked for the advice. However, itsmoked you are disagreeing about the number of SSR's! compositepro says 2 SSR+ relay, you say 1 SSR + relay. why? If its normal can you refer me to an example.

 

[Compositepro]

No, I meant that one would do, and explained why two were not needed, but if you really want to use a two pole switch for some reason, it can be accomplished with two single pole switches.

Since you seem so new at this, please pay particular attention to what we said about over temperature protection. The damage caused by out-of-control and unattended heaters can be extremely serious and, eventually, pretty likely to happen.

 

[petervb]

OK, understand CompositePro

 

[danw2]

Great engineerirng guide for SSRs from Omron in the attachment.

Here's a graphic for what Compositepro is explaining about a limit controller. The safety contactor really is critical.

 

[MacGyverS2000]

I wish eng-tips had a macro that autocorrected 110/220V to 120/240V automatically

Dan - Owner

http://www.Hi-TecDesigns.com

 

[moltenmetal]

The alarm trip that danw2 helpfully posted the graphic for, is a requirement for any heater which can do damage to more than just the heater if it is left 100% on for long periods of time, or in some upset state that will cause problems for more than the heater. It's prudent but unnecessary as a means to just protect the heater from burning out if it doesn't cause any other harmful consequences- that's an equipment protection rather than a safety issue. The requirement is for the 2nd (mechanical) safety contactor, to protect against the case when the solid state relay melts into a solid conductor and leaves the heater 100% on, which is a known failure mode. Independent temperature measurement and separating the trip function form the controller raises the safety integrity level further but is only required when a LoPA analysis says that it is- what IS required in the conditions noted above is a means to ensure that the power to the heater can be reliably shut off when the solid state relay fails.

 

[jraef]

So to add clarity and raise one more issue:
There is no reason to use SSRs in both poles from a functional standpoint, the control of heat will be the same whether it is one pole or two. But it is slightly more reliable in that if one SCR shorts, you still have control. Cheap ovens for cooking pizzas or sandwiches in small restaurants will usually only use one pole control to keep costs down, because the consequences of runaway heat becoming dangerous are low since they are usually attended and designed with a separate On-Off controller which acts as that safety contactor if anything goes wrong. But if the machine is making something that generates a lot of revenue and down time is going to result in losses, that second pole of control means a continued revenue stream, so the cost of it is immaterial.

The other issue is that the fused switch or circuit breaker PROTECTING this equipment must still be 2 pole. It's OK to control with only one pole, but you must PROTECT both circuit conductors.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[LionelHutz]

I would also recommend the safety contactor be 2 pole, especially if using fuses for protection. Otherwise, if a short to ground happens on the output of the switched pole the heaters could receive 1/2 voltage that is not controlled and won't get turned off by the over temperature safety circuit.

 

[waross]

I used to encounter a lot of single pole contactors in 208/240 Volt service in economy priced A/C units.

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

 

[jraef]

The effort expended to build things as cheaply as possible that, from an engineering perspective, seem outright foolish, never ceases to be amaze when I come across it.

A few years ago I found out that elevators that use Wye-Delta controllers routinely use only 2 contactors instead of 3, meaning one side of the motor circuit is permanently connected to the line. That means of course that one welded contact anywhere else results in unrestricted current flow to that winding and the likely loss of the motor. They don't seem to care.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington