QualifiedMedium risk

Heater bank drawing uneven current across phases

A multi-element heater bank pulls noticeably different current on each phase. Heating is uneven, output is low, or a phase reads much lower than the others.

Start fault-finding

Safety first

Heater banks run hot and at full load current. Prove dead and allow elements to cool before touching. Treat all phases as live until isolated and proven.

Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.

Full detail — causes, the why, and common mistakes.

Likely causes

Ranked from most to least likely.

1
One or more open (failed) elements
Most likely
A burnt-out element stops drawing current on its leg, dropping that phase's total and unbalancing the bank.
2
Open SSR / contactor pole feeding one phase
#2
If the switching device for one phase isn't conducting (failed SSR, burnt contact), that whole phase's elements get nothing.
3
Loose or high-resistance connection
#3
A loose terminal or corroded joint adds resistance on one leg, lowering its current and creating local heating.
4
Element drifted in resistance / partial degradation
#4
Aged elements can rise in resistance before they fail completely, pulling slightly less current and unbalancing the bank.
5
Supply imbalance feeding the bank
Least likely
An unbalanced or partially lost supply phase reduces current on that leg independent of the elements themselves.

Reports are saved on this device to reflect what you actually find.

Testing sequence

Work through one test at a time. Expected reading and what each result means.

Test 1 of 4

Clamp the current on each phase feeding the bank and compare the three readings.

Expected reading

Three roughly equal currents consistent with a balanced resistive load.

Your actual reading

If it passes

Currents balanced — if heating still seems off, look at control/setpoint, not the bank.

If it fails

An imbalance confirms the fault. Note which phase(s) are low and by how much.

Often leads toSolid-state relay (SSR) stuck on — heat won't switch off Three-phase equipment single-phasing (lost a phase)

View all expected readings at once

1. Clamp the current on each phase feeding the bank and compare the three readings.

Three roughly equal currents consistent with a balanced resistive load.

2. Check the switching device (SSR/contactor) for the low phase is actually conducting when called.

Full voltage passed through to the elements on the low phase when switched on.

3. Isolate, lock off, and measure each element's resistance (and check terminal tightness).

Elements on each phase read similar, sensible resistances; terminals tight.

4. Measure the incoming supply voltage balance across the three phases at the bank.

Roughly balanced phase voltages.

Fault-finding flowchart

The same logic as a decision tree.

1
start
Heater bank uneven current
→ step 2
2
decision
Is one (or more) phase current clearly low?
Yes→ step 3No→ step 4
3
decision
Is the SSR/contact for the low phase conducting when called?
Yes→ step 5No→ step 6
4
result
Currents balanced — look at control/setpoint, not the bank.
5
decision
Are the elements/connections on that leg healthy?
Yes→ step 7No→ step 8
6
result
Non-conducting SSR/contact on that phase — replace/repair.
7
result
Check supply voltage balance feeding the bank — imbalance is upstream.
8
result
Open/degraded element or loose joint on that leg — repair/replace.

Common mistakes apprentices make

Measuring total current only and missing which phase is low.
Condemning elements without checking the SSR/contact for that phase is actually conducting.
Overlooking a loose terminal that's adding resistance and heating locally.
Assuming the elements when the supply itself is unbalanced.

When to stop & escalate

If multiple elements have failed or the bank is aged and drifting, plan a coordinated replacement rather than chasing one element at a time. Supply imbalance traced upstream of the bank should be investigated on the distribution, not the heater.

If you're past your competence, authorisation, or the safe limits of the job — stop and hand it on. There's no fault worth getting hurt over.

Related faults

Heating elements & SSRs Premium

Solid-state relay (SSR) stuck on — heat won't switch off

A heater (or other SSR-driven load) stays on even when the controller commands it off. Temperature overshoots, or the load runs continuously regardless of the control signal.

4 causes3 test steps

AdvancedHigh risk

General testing

Three-phase equipment single-phasing (lost a phase)

Three-phase equipment is misbehaving — motors humming, struggling, overheating, or tripping — because one phase has been lost somewhere between the supply and the load.

5 causes3 test steps

QualifiedHigh risk

Motors & overloads

Motor overload keeps tripping

The thermal/electronic overload trips repeatedly, either on start or after the motor has run for a while. Resetting only buys you a short run before it trips again.

5 causes4 test steps

QualifiedHigh risk