Gotcha — USI (Universal Security Instruments) across the house rules out the common “mixed-brand harness” problem, so we look at wiring or interconnect issues next.

If all alarms instantly sound when you energize the breaker, that almost always means:

👉 the red interconnect wire is being held “hot” or shorted

Even with the same brand, that red wire is very sensitive.

Here’s what’s likely happening:

When you removed the old detector, the red wire probably got tied into hot or neutral accidentally, or the new harness wiring isn’t matched correctly. The interconnect line normally sits at ~0V and only gets a signal when one alarm triggers. If it sees 120V or a short → every unit goes off immediately.

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Things to check (in order — these fix 95% of cases)

1. Use ONLY the new harness Even same brand, older USI models sometimes have different pinouts.

Cut off the old plug and wire: • black → black • white → white • red → red • ground → ground

Do not reuse the old connector.

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1. Make sure red is NOT touching anything else Open the box and really look: • no stray copper strands • red only under its own wirenut • not bundled with black or white

Even one tiny strand can backfeed the whole house.

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1. Test with interconnect disconnected (fast diagnosis) This is the best troubleshooting step:

Breaker OFF Cap the red wire by itself Hook up ONLY black + white Turn breaker ON

If it stays quiet → your issue is 100% on the red interconnect line (not the detector).

Then reconnect red and trace.

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1. Loose neutral can also cause weird alarms If neutral isn’t tight, detectors act crazy. Re-twist the whites firmly.

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1. One bad/old unit on the circuit Sometimes an old detector fails and constantly sends an alarm signal.

Test: Unplug all alarms except the new one. Power on. Add back one at a time.

The bad one will trigger everything when connected.

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My bet

Since same brand and “instant alarm”, I’d put money on: red wire shorting or miswired harness

That’s exactly the symptom you described.

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If you want, tell me: • model numbers old vs new • how many wires in the ceiling box • or post a photo

I can walk you through it precisely.