# 3 pole SSR with Neutral connection

I am designing a heater block using 12 heaters. Each one has 200W/230V ratings. I have decided to use 3 pole SSR for control heaters with help of PID temperature controller.

The SSR manufacture recommend connection method as below image. (Star or delta connection).

Manufacture recommended connection method

My connection method

But my heater voltage is 230V and I decided to use 4group of parallel connected heaters for each pole of the SSR. (Local phase to phase voltage is 400V and phase to neutral is 230V)

My question is "Is there any problem of my connection method relative to manufacture recommended(Actually not recommended. But its the mentioned method) method?"

• The diagram doesn't actually indicate unambiguously how you plan to make connections. You don't have links to the devices either so nobody is going to tell you it's good without checking all these things. Nov 16, 2020 at 19:13
• You should use the 'Star' (aka Wye) connection, and if your heaters are arranged so that you have the same load on each phase then you don't need to connect neutral (assuming you have a 3-phase supply). Nov 16, 2020 at 19:32
• @brhans Its correct. Dont need a neutral connection for Star. But my heaters rated voltage is 230V. If I connect as Star I can only connect 9 heaters. I have 12. And also the heater voltage will 200V. Its less than rated value. Nov 16, 2020 at 19:38
• @Andyaka I have added a simple sketch of my connection method and edited the question. Nov 16, 2020 at 19:39
• What makes you think that you can only connect 9 heaters in Star? The connection for 12 is exactly as you've drawn it (3 banks of 4 in parallel) except that the Neutral connection is not required. Each heater will see the 230V phase voltage. Nov 16, 2020 at 19:41

You left out a ton of details and have probably overlooked one of following.

• SSR's are often ZCS (zero crossing switch) type so that won't work well with PID control.(that need phase controlled Triacs or missing pulse control)
• you won't need PID control (anyhow) if the sensor is accurately positioned close but not too close to (/far from) the heater so as not to have a time delay with under(/overshoot. )
• Just use ON/OFF control with a setpoint temperature-controlled voltage.
• 750W/1500W/3kW block heaters are standard for oil or water here. (N.A.)
• Why did you choose 200W x 12 = 2400W?
• your thermal time constant may burn out the heater elements if dry in a few minutes, there are current sensing methods to detect this condition since the resistance rises.

It's not that dumb of a question. As you saw, your proposal is the same as the 'star' (wye) connection.

What does neutral do, then? Neutral comes in to play when there is an imbalance in the phases. In your case, the current is balanced, so neutral isn't necessary. In the wye connection the net voltage on the center terminal is zero.

That said, you should consider a delta connection instead. It has better fail-safe properties than wye.

• @user_fs10 can't connect 230 V heaters in delta on a 230/400 V supply; they have to be connected in star. A delta connection will require 400 V heaters. The only question is whether a neutral is provided. A star connection will work without one but providing a neutral connection will safeguard the system from the effects of a heater burning out and raising the voltage on the other heaters on that phase (which will also then burn out). The SSR should of course be supplied from a 3 phase MCB (4 pole if neutral is used). Tony Stewart's point on PID control is also important. Nov 17, 2020 at 0:25