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IES Systems Software Engineer Interview Guide

IES Systems Software Engineer interview guide: reported rounds, questions and how candidates describe the process.

The 60-second brief

At IES Systems, the Software Engineer (often functioning within an Electrical or Mechanical engineering capacity) plays a critical role in the safety and reliability of industrial machinery. You are the architect of protection, tasked with designing, analyzing, and implementing safety systems that safeguard both human operators and high-value manufacturing equipment. Your work directly impacts how our clients maintain compliance with global standards while ensuring maximum uptime and operational efficiency. This role is unique because it bridges the gap between high-level design and hands-on, field-based execution. Whether you are creating electrical schematics in the office or performing a hands-on safety validation at a customer’s manufacturing facility, your influence is foundational.

This guide is scoped to a Software Engineer candidate at IES Systems.

Focus areas

  • Machinery Safety Systems
  • Risk Assessment Documentation
  • Safety Systems Engineering (Design and Analysis)

The IES Systems interview process

IES Systems candidates report 4 rounds over 3-5 weeks. The stages below are what candidates describe, not a published process.

Application Review

Initial assessment of your application to determine fit for the role.

What to demonstrate

  • Initial assessment of your application to determine fit for the role
  • Depth in Machinery Safety Systems

How to prepare

  • Be able to walk your CV end to end in two minutes, and say why this company specifically.
  • Have your salary expectations, notice period and location constraints ready, and ask for the rest of the loop in writing.

Technical Screen

Discussion focused on technical skills and knowledge relevant to the position.

What to demonstrate

  • Discussion focused on technical skills and knowledge relevant to the position
  • Depth in Machinery Safety Systems

How to prepare

  • Answer aloud and timed: What is your experience with ISO/IEC/ANSI machine safety standards, such as ISO13849-1?
  • Answer aloud and timed: How do you determine if a machine requires light curtains versus physical interlocks?

Situational Assessment

Evaluation of your ability to handle real-world scenarios related to engineering and client interactions.

What to demonstrate

  • Evaluation of your ability to handle real-world scenarios related to engineering and client interactions
  • Depth in Machinery Safety Systems

How to prepare

  • Answer aloud and timed: Describe your experience with Safety PLCs and the programming or integration process.
  • Answer aloud and timed: Describe a time you had to troubleshoot a safety system failure in the field.

Final Evaluation

Comprehensive review of your performance throughout the interview process before a decision is made.

What to demonstrate

  • Comprehensive review of your performance throughout the interview process before a decision is made
  • Depth in Machinery Safety Systems

How to prepare

  • Answer aloud and timed: How do you handle a situation where a customer’s facility does not meet the safety requirements you’ve identified?
  • Answer aloud and timed: How do you balance the need for safety with the production requirements of a client?

Where candidates lose points

Prepare your portfolio

Even if you cannot share proprietary designs, be ready to discuss the types of schematics you have produced and the complexity of the systems you have managed.

Focus on the "Why

Don't just say you used a specific safety standard; explain why that standard was the right choice for that specific machine application.

Emphasize independence

Since you will be working at customer sites, highlight examples where you had to make a critical decision on your own to keep a project moving safely.

Know the hardware

Be prepared to talk about your hands-on experience with specific safety components like scanners and light curtains; we value candidates who understand how to wire and install what they design.

Questions & practice

How do you balance the need for safety with the production requirements of a client?

Medium · Field Experience & Problem Solving

Approach

  1. Restate the input: its shape, its size, and what is guaranteed about it.
  2. Name the brute-force solution and its complexity before improving on it.
  3. Choose the data structure from the access pattern, not from familiarity.
  4. State the target complexity and say which constraint rules the naive version out.

Follow-up

  • How does this change if the input no longer fits in memory?
  • What is the worst case, and how likely is it on real data?

What is your strategy for documenting a risk assessment while on-site?

Medium · Field Experience & Problem Solving

Approach

  1. Restate the input: its shape, its size, and what is guaranteed about it.
  2. Name the brute-force solution and its complexity before improving on it.
  3. Choose the data structure from the access pattern, not from familiarity.
  4. State the target complexity and say which constraint rules the naive version out.

Follow-up

  • How does this change if the input no longer fits in memory?
  • What is the worst case, and how likely is it on real data?

Diff a projection against the primary without per-row point reads

Hard · reconciliation · range hashing · throttling

The listing projection has drifted and some rows show a stale version. The primary holds 40,000,000 resource rows across 12,000 tenants while serving 1,200 writes and 14,000 reads per second. The obvious repair, reading each resource row and comparing its version against the projection, is correct and would eventually finish. Explain precisely why it is unacceptable here, then give a diff that finds the differing rows, state its complexity, and make it safe to run against a live primary. Replication lag is usually under 100 ms and is not bounded.

Approach

  1. Quantify the naive cost rather than calling it slow: 40,000,000 point reads at even 0.5 ms each is over five hours serialised, and the only lever is concurrency, which is exactly what you cannot spend. The primary's pool is sized for the write path, and 40,000,000 random reads evict the buffer cache that sustains the 85 percent cache hit rate, so the audit degrades the system it is auditing.
  2. Replace random access with one ordered pass per side. Both sides can be read in (tenant_id, resource_id) order, which is a sequential scan on each and a merge join in O(n) time and O(1) memory. For a dense diff that is the whole answer, and it reads the primary once instead of 40,000,000 times.
  3. For the expected sparse case, compare range hashes instead of rows: partition the key space, compute per range an order-independent aggregate over hash(resource_id, version), compare aggregates, and descend only into ranges that differ. With d differing rows and branching factor B, at most d ranges mismatch per level, so the drill-down examines O(d log_B(n/d)) ranges and reads full rows only in mismatching leaves.
  4. Aggregate with a sum modulo 2^64 or a multiset hash, never XOR. XOR is order-independent but self-cancelling, so two rows wrong in the same way, or a row duplicated on one side, leave the range aggregate matching and the range is declared clean.

Follow-up

  • The diff reports 900 stale rows. How do you decide between patching those rows and rebuilding the projection from resource_revision?
  • Same job, but the projection lives in a search index that cannot be scanned in key order. What changes?

Find version gaps and relay lag with window functions

Medium · window functions · event tables · frame semantics

outbox_event holds event_id, aggregate_type, aggregate_id, aggregate_version, event_type, payload, status ('pending','published','dead'), attempts, created_at, published_at. A projection is missing rows and you must decide whether the relay skipped events or the consumer dropped them. Write three queries over the last seven days: one listing every aggregate_id whose published aggregate_version sequence has a hole, one giving per-day counts with a running total, and one returning the newest published event per aggregate. For each, say where the window function is evaluated relative to WHERE and LIMIT. PostgreSQL 16.

Approach

  1. Gaps: compute lead(aggregate_version) OVER (PARTITION BY aggregate_id ORDER BY aggregate_version) in a subquery, then filter next_version <> aggregate_version + 1 in the outer query. Window functions are evaluated after WHERE, GROUP BY and HAVING and before the outer ORDER BY and LIMIT, so the predicate cannot sit in the same WHERE clause and PostgreSQL 16 has no QUALIFY.
  2. Say what the seven-day filter does to the answer: it truncates every partition, so the first row per aggregate has no predecessor inside the window and a hole spanning the boundary is invisible. Widen the window, or join to resource.version as the authority for the true maximum.
  3. Running total: SELECT date_trunc('day', created_at) AS d, count() AS n, sum(count()) OVER (ORDER BY date_trunc('day', created_at) ROWS UNBOUNDED PRECEDING). An aggregate inside a window call is legal because grouping runs before windowing. The grouping key is unique per row here so ROWS and RANGE agree, but write the frame anyway — over ungrouped rows with tied timestamps the default RANGE frame pulls in every peer row and the total jumps.
  4. Newest per aggregate: DISTINCT ON (aggregate_id) ... ORDER BY aggregate_id, aggregate_version DESC is the cheap PostgreSQL-only form when an index matches that order; row_number() OVER (PARTITION BY aggregate_id ORDER BY aggregate_version DESC) = 1 is the portable form and needs a subquery for the same evaluation-order reason as the gap query.

Follow-up

  • Relay failover redelivers events. Does a duplicate break the gap query, and how would you detect one from this table alone?
  • Turn the gap check into a continuous monitor rather than a query someone runs after an incident. What does it watch?

Stop tag and share joins from fanning out a page

Medium · join fan-out · semi-joins · many-to-many

resource_tag is (resource_id, tag_id) with PK (resource_id, tag_id); resource_share is (resource_id, shared_with_user_id, permission). The tagged-and-shared listing inner-joins resource to both, filters tenant_id, tag_id = ANY($2) and shared_with_user_id = $3, orders by updated_at DESC and takes 50. Pages come back with fewer than 50 distinct resources and the total in the header is far too high. Explain the row multiplication, rewrite both the page query and the count query so each is correct, and name the index each one needs. PostgreSQL 16.

Approach

  1. Do the arithmetic against the predicates that are actually there. An inner join emits one row per matching child row, and both joins are filtered: tag_id = ANY($2) admits only the requested tags, shared_with_user_id = $3 admits one user's share rows. So a resource holding three of the requested tags and shared with $3 once yields three rows, not one — the multiplier is its count of matching tags times its share rows for that single user, and that second factor is 1 unless the table admits duplicate (resource_id, shared_with_user_id) pairs. LIMIT 50 then limits rows rather than resources, and COUNT(*) counts pairs — the header is the product, not the population.
  2. Reject DISTINCT as the fix. It deduplicates after the product has been built, so the planner must materialise and sort the fanned-out set before the LIMIT can apply, and it leaves any SUM or AVG in the same select list wrong.
  3. Rewrite both filters as semi-joins, keeping resource as the only row source: AND EXISTS (SELECT 1 FROM resource_tag rt WHERE rt.resource_id = r.resource_id AND rt.tag_id = ANY($2)) and the same shape against resource_share. A semi-join stops at the first match per resource and preserves the driving index order, so ORDER BY updated_at DESC, resource_id DESC LIMIT 50 still stops after 50 rows.
  4. Count with the same predicates and no join at all: SELECT count(*) FROM resource r WHERE r.tenant_id = $1 AND r.status = 'active' AND EXISTS (...) AND EXISTS (...). Nothing multiplies a resource, so the number is the population.

Follow-up

  • The filter changes from 'any of these tags' to 'all of these tags'. Rewrite it and state what it costs relative to the ANY form.
  • A resource can be shared with the same user twice under different permissions. Does your count change, and should it?

How do you approach the design of a safety system for a piece of legacy equipment?

Medium · Technical Proficiency & Standards

Approach

  1. Clarify what is being asked and what a complete answer contains.
  2. State your assumptions explicitly before working the problem.
  3. Say what you would check first and why it is the highest-information step.
  4. Work from the requirement backwards to the design.

Follow-up

  • What assumption would you test first?
  • How would you know your answer was wrong?

Can you explain your process for creating and reviewing electrical schematics?

Medium · Technical Proficiency & Standards

Approach

  1. Clarify what is being asked and what a complete answer contains.
  2. State your assumptions explicitly before working the problem.
  3. Say what you would check first and why it is the highest-information step.
  4. Work from the requirement backwards to the design.

Follow-up

  • What assumption would you test first?
  • How would you know your answer was wrong?

How do you determine if a machine requires light curtains versus physical interlocks?

Medium · Technical Proficiency & Standards

Approach

  1. Clarify what is being asked and what a complete answer contains.
  2. State your assumptions explicitly before working the problem.
  3. Say what you would check first and why it is the highest-information step.
  4. Work from the requirement backwards to the design.

Follow-up

  • What assumption would you test first?
  • How would you know your answer was wrong?

Every query on one table stalls for forty seconds mid-deploy

Medium · lock contention · ddl · migrations

During a release on PostgreSQL, every query touching resource times out for about 40 seconds and then recovers with no intervention. The release ran one migration, ALTER TABLE resource ADD COLUMN archived_reason TEXT, and the migration log shows it completing in 6 ms. Unrelated tables showed no change in error rate. Explain how a 6 ms statement caused a 40-second stall, give the ordered checks you would run on a live system to confirm it, and give the migration procedure that prevents a repeat.

Approach

  1. Separate the statement's duration from the lock's duration. ADD COLUMN with no default is a catalogue-only change and genuinely runs in milliseconds, but it requires ACCESS EXCLUSIVE, and it cannot acquire that until every transaction already touching the table has finished.
  2. Account for the queueing, which is the part that surprises people. A lock request that is waiting blocks later requests for conflicting modes behind it rather than letting them overtake, so one long-open transaction holds the DDL and the DDL holds all the traffic. The stall length is set by the longest open transaction, not by the size of the change.
  3. Confirm on a live system in this order: pg_stat_activity for that table ordered by xact_start, looking for the oldest transaction and specifically for state = idle in transaction; then pg_locks where granted = false to find the waiter; then join them on pid to name blocker and blocked. pg_blocking_pids() does that join for you and is the fastest single call.
  4. Prevent rather than merely time it better. Set lock_timeout to a second or two on the migration session so the DDL abandons the queue after a bounded wait and is retried, instead of holding it for as long as the oldest transaction lives. Be exact about what that buys: queries arriving during the wait still queue behind the pending ACCESS EXCLUSIVE request, so each attempt costs them up to one lock_timeout of added latency. The outage goes from 40 seconds to about one second per attempt, not to zero. Also run migrations away from deploy-time peaks, and put a statement timeout and an idle-in-transaction timeout on the analytics role that opens the long transactions.

Follow-up

  • The same release also wants NOT NULL on that column. What is the sequence that gets there without a long lock?
  • Your lock_timeout retry fails ten times in a row because the analytics transaction is always open. What do you change?

Suggested practice checklist

Built from the rounds and topics IES Systems candidates report.

Day 1 — Map the IES Systems loop

  • Write out the reported sequence: Application Review, Technical Screen, Situational Assessment, Final Evaluation.
  • For each round, write one sentence on what it is judging, from the description above, and mark the one you are least ready for.

Deliverable: A one-page map of the 4 reported rounds, with the weakest marked.

Day 2 — Work Machinery Safety Systems

  • Spend the session on Machinery Safety Systems, which IES Systems candidates report being tested on.
  • Write one worked example in Machinery Safety Systems and time yourself on it.

Deliverable: One timed worked example in Machinery Safety Systems.

Day 3 — Work Risk Assessment Documentation

  • Spend the session on Risk Assessment Documentation, which IES Systems candidates report being tested on.
  • Write one worked example in Risk Assessment Documentation and time yourself on it.

Deliverable: One timed worked example in Risk Assessment Documentation.

Day 4 — Work Safety Systems Engineering (Design and Analysis)

  • Spend the session on Safety Systems Engineering (Design and Analysis), which IES Systems candidates report being tested on.
  • Write one worked example in Safety Systems Engineering (Design and Analysis) and time yourself on it.

Deliverable: One timed worked example in Safety Systems Engineering (Design and Analysis).

Day 5 — Answer out loud: Technical Proficiency & Standards

  • Answer aloud, timed: How do you approach the design of a safety system for a piece of legacy equipment?
  • Answer aloud, timed: Can you explain your process for creating and reviewing electrical schematics?

Deliverable: Spoken answers to 2 reported Technical Proficiency & Standards question(s), under time.

Day 6 — Answer out loud: Field Experience & Problem Solving

  • Answer aloud, timed: Describe a time you had to troubleshoot a safety system failure in the field.
  • Answer aloud, timed: How do you handle a situation where a customer’s facility does not meet the safety requirements you’ve identified?

Deliverable: Spoken answers to 2 reported Field Experience & Problem Solving question(s), under time.

Day 7 — Dry run for IES Systems

  • Run one full mock under time, then write down the two questions you most want to ask your interviewers.

Deliverable: A completed timed mock and two questions to ask.

Behavioural preparation

Behavioural rounds judge the decision you made and what it cost.

What is your experience with ISO/IEC/ANSI machine safety standards, such as ISO13849-1?

Medium · Technical Proficiency & Standards

Approach

  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.

Follow-up

  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?

Describe your experience with Safety PLCs and the programming or integration process.

Medium · Technical Proficiency & Standards

Approach

  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.

Follow-up

  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?

Describe a time you had to troubleshoot a safety system failure in the field.

Medium · Field Experience & Problem Solving

Approach

  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.

Follow-up

  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?

How do you handle a situation where a customer’s facility does not meet the safety requirements you’ve identif

Medium · Field Experience & Problem Solving

How do you handle a situation where a customer’s facility does not meet the safety requirements you’ve identified?

Approach

  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.

Follow-up

  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?